Of the two its styling is the most exciting, commensurate with its younger target market.
From the front there's a wide macho look, with a wide front track that reminds one very much of the "arrow head" epitomised by Honda's S2000 sports car.
The under-bumper area is particularly interesting, with a deep air intake flanked by sculpted brake ducts inset with round foglamps.
The side aspect sees with a sweeping line which rises as a crease along the length of the vehicle and culminating at the tail, with, like the GS, a fastback shape.
The rear end has strong taillights and large exhaust pipes at each extremity.
While lithe and dynamic on the outside, the IS250 is not only that on the inside, but also luxurious.
The dashboard flows softly towards the driver before cutting back to form the instrument binnacle, and the door trim has a unique configuration with a raised surface running rear to front that rapidly changes direction (creating an S shape) to form the door grip.
Of particular interest in the IS250 is the interior lighting.
The IS250 illuminates in a very white light thanks to LED interior spot lighting. There is a combination of ambient and map reading lights, and the SE model has an illuminated scuff plate.
The three-dimensional Optitron instrumentation lights up just the needles at start-up and these whip around the clock in welcome.
The speedometer has a speed warning device that lights up an orange ring inside the instrument when a set speed is exceeded. A similar system is used on the rev counter.
The sculpted electric seats (with memory on SE derivatives) are covered in leather as standard.
Smooth leather is used on the base model and a perforated treatment on the SE. The rear seat has a centre armrest with a cubby hole and drinks holders that also allows access through a ski-hatch to the boot.
Specification levels are very comprehensive as standard. The base model rides on 16 inch alloy wheels shod with 205/55 tyres and has fog lights front and rear. While the headlights are halogen units, they look exactly like the High Intensity Discharge (HID) headlights of the SE model.
Keyless entry is standard with a start/stop button on the dashboard and the interior is trimmed in smooth leather.
There are one-touch electric windows all-round, electrically heated mirrors, dual zone automatic climate control with a pollen filter, eight-way adjustable electric seats with lumbar support, a multi-function steering wheel that is manually adjustable for reach and height, and a six-CD front loader audio system with 13 speakers and an auxiliary wireless drive for an iPod or MP3 player.
There is a windscreen de-icer and automatic halogen headlights as well as remote releases for the fuel flap and boot lid.
Cruise control is standard, controlled on the steering column, and there is an on-board computer displayed in the instrument binnacle with various functions controlled from the steering wheel.
The SE model rides on 17 inch alloy wheels shod with 225/45 tyres on the front and 245/45 tyres on the rear.
The SE designation adds Park Distance Control (PDC) front and rear with a rear-facing camera, AFS (Adaptive Front-lighting System), HID headlights with headlight washers and automatic levelling, automatic windscreen wipers with a rain sensor, electro-chromatic mirrors, seat heaters and coolers, three seat memories, electric adjustment for the steering wheel, and an electric rear blind.
The sound system is upgraded to a 14-speaker Mark Levinson digital surround sound system and the SE model benefits from the addition of EMV with navigation and Bluetooth connectivity.
There is a 17.8 cm LCD touch-screen system with voice recognition.
The IS250 is powered by a 2.5-litre quad cam 24-valve V6 engine similar to that in the GS300.
It develops 153 kW at 6 400 r/min and 252 Nm of torque at 4 800 r/min. The torque curve is very flat with virtually all its torque available from 2 000 r/min all the way to 6 000 r/min.
The rear wheels are driven through a choice of either a manual or automatic transmission, both six-speed gearboxes.
The automatic transmission features shift control that adapts to the road conditions and the driver's driving style. The staggered gate operation features a Sport mode with manual driving by a sequential shift mode on the gear lever, or by steering wheel mounted paddles.
There is a display on the dashboard to indicate the selected gear.
The manual IS250 can accelerate from rest to 100 km/h in 8.1 seconds and on to a top speed of 230 km/h. The automatic's figures are 8.4 seconds and 225 km/h respectively. Claimed overall fuel economy is 9.3 litres/100 km and 9,9 litres/100 km respectively.
As with the GS300 ride and handling are excellent, combined with super ride comfort.
Extensive use of aluminium has reduced unsprung mass for better wheel control and vibration reduction.
The front suspension is independent by double wishbones, while the rear employs a multi-link set-up. Anti-lift and anti-squat geometry has been used to improve body control under acceleration and braking.
The IS250 gets speed sensitive electric power steering.
Safety features include ABS, EBD and (BAS) as well as traction control (TRC) and VSC stability programme.
While TRC can be disengaged by a switch on the dashboard when required, VSC remains constantly active.
Dual-stage front airbags, knee airbags, seat mounted side airbags and inflatable curtain airbags are standard.
On the road
The previous IS300 was lithe and very balanced, and although the latest car has grown somewhat, it's lost none of this.
I drove the manual car first, with wet roads and some mist dampening things outside, though definitely not INSIDE the car.
The best thing about bad conditions is that they allow you to find the car's limits sooner, and I was very impressed, again, by the way the cars hold the road without trying to send your kidneys to marshmallow heaven.
On one particularly tight hairpin on Fransschoek Pass I deliberately went in way too fast to see how the car would respond; it just shrugged those beefy shoulders, the VSC kicked in, and with a couple of chirrups from the front and rear tyres we were on our way.
The gear change was especially joyful, and it was with some trepidation that I approached the auto version.
But I needn't have worried.
Steering wheel paddles allow you more control as you can keep both hands on the wheel while changing gear.
And joy of joys - the gearbox stays in the gear you're selected until you change it, allowing the same precision of control as the manual.
Quite frankly I'm getting fed up with German auto gearboxes which change up halfway through a corner, when you'd prefer the engine to be revving high in preparation for maximum power as you exit the bend.
Although the GS300 SE's haven't arrived yet, the auto IS models were SEs, and so came with the rear camera.
This is a great feature, for it allows you to see right behind the car, at number plate level, so parking is extremely precise, especially since Lexus thoughtfully provides you with guidelines so you can even estimate the car's overhangs.
Like the GS300 the IS250 comes with a 4 years/100 000 km maintenance plan.
Prices: IS250 manual R274 000; auto R287 000; IS250 SE manual R329 100; auto R342 100.
Both these cars are going to shake up the market.
Although on the face of it they appear to be slightly more expensive than their natural opposition, when you examine what they get as standard compared to the long list of "options" you need to get the BMWs and Mercs up to the same level you realise that the value proposition is very high.
Lexus marketing boss Brian Hastie says the whole Lexus network is taking a shift in direction, with stand alone showrooms and workshops being built, and the accent very definitely towards providing the sort of sales and service experience that continually makes Lexus top dog in the JD Power customer satisfaction surveys.