When I started looking, I thought it was a forgone conclusion I'd get the X3. But after researching it against the others and driving both X3 and a Q5, I prefer the Q5. So go ahead and drive them.
From a driving dynamics view, the Q5 brakes faster but you can get DHP on the X3 without having to buy the top model like you do on the Q5. Also, the roof rack on the Q5 is intelligent in that it detects a load and modifies the car's suspension/handling dynamics according to the shift in COG. If you want, you can get V6 performance with a 4 cylinder with the Q5 Hybrid power train but it's only available in Audi's top trim line. It adds about $4000 over the equivalent trimmed V6.
I preferred the ride (noise, adjustable backseat, seat comfort and handling) of the Q5. Steering to me was equivalent.
If Navi is important, the Q5 is quite better with voice enabled google search and google 3D maps. Unlike the current X3, the Navi displays the street names. I'd wait for the 2014 X3 Navi if it's important to you. I don't like having to pay for the M-sport on the X3 to get a masculine front end yet still have to suffer the AMC Gremlin-esq rear quarter window and mis-aligned tail lights. But beauty is in the eye of the beholder. There is no perfect vehicle... yet. I'm looking forward to seeing and test driving the 3 series Gran Tourismo tho, that may score pretty well. YMMV