Audi:
I worked at a vw/audi dealership before, and all i can tell you is that the Q5 is far better of a car than the RX!! Compared to the RX, the Q5 feels more livelier - the steering feel is way better, both the pedal feel is supurbly better, and the engine response is respectable.
if you want a sportier ride, go for the 2.0t Q5, not only is it cheaper, it does better on gas and you'd be surprise how hard it actually pulls. want a little more power? chip it and it can easily go up 50hp or so, since it adds boost to the turbo, if you dont step on it, it will not disturb your gas mileage at all.
3.2 feels more comfortable, offers a smoother ride, but it doesnt match up to the sporty feel of the 2.0t
HOWEVER, the cons with the audi is reliability. Yes, they're well made, fit and finish is supurb (matching that of bmw/benz if not better), material top notch. BUT it is not at all the most reliable car you can get. random stuff goes wrong with these cars that are unforseen. [from what i've seen in the dealership, new cars can come back time after time fixing the SAME unexplainable problems]. THAT being said, some audi owners never experience any problem whatsoever. In fact, when compared to previous models of vw/audi, their reliablility has largely improved, but still could not be compared with that of the japanese. at the end of the day, buying an audi is like playing russian roulette, if you're not lucky, be prepared to spend countless visits to the shop.
(keep this in mind though, most european cars ARE like this, whether its bmw or the merc, jaguar, ferrari, lambo, you name it~ its just that people who owns those complaints less about it because they are a more premium brand >=] )
Lexus:
I have also have a chance to drive the RX, although the steering/pedal feel is basically lifeless, the engine does pull quite hard!!! there is no time on the drive that you'll feel you don't have enuf beef from the engine!
the RX also offers MORE space for all the passengers and WAY more luggage space (the Q5 has SOOOO little space you'd get more out of any sedan [excluding compact ones]) so overall it is a more comfy car (at least for the passengers).
Lexus is always rated the best in reliability. i have a 97 es300 in the family with approx 200k miles (ALL city driving [worse for cars]) and never had a problem with it.
So if you don't mind sacraficing the driving experience for comfort, the RX is better of the two. Sure, the light gas pedal takes a couple days to get use to, it has a more plastic'ky interior and the design that couldnt match audi, and that it actually does quite poorly on gas mileage, and to top it off the base price is $4000 more for AWD where as the Audi comes standard.
IF i have the choice, i'll be bias and pick the Audi, it has better styling (subjective), better handling, high quality interior, better driving dynamics all round. but reliability is the trade off.
however, it does make sense to go with the RX because it serves its purpose better. a family vehicle that is both more reliable, spacious, relatively cheaper to repair (if anything does go wrong), and most of all, outstanding comfort for all passengers. afterall, if you want to go sporty, SUV isnt really the ride you should get - just jump on the NSX
try both of them out, in a way, the are both SUVs, however, they are completely different cars. choose the one that suits your lifestyle more