I saw the 5th gear episode last night re R8 Spyder vs SL63, given i have an R8 i was more than pleased it won...
In terms of Q5, i don't recall seeing a comparison tbh.
At the risk of offending some i'll just say i find the demographics of the buyers in terms of the two cars somewhat different.
Merc people tend to be older. I don't dislike it, but i never considered one - it just never appealed.
I was seriously looking at the C220 CDI Estate AMG Sport as a work horse before I binned my BMW 135i and decided to get a fun workhorse in the SQ5, and it was a lovely car, but found the ergonomics awful.
They still have a button for EVERYTHING, rather than menu driven etc. I can only assume (and am jumping on Snowmans say stereotyping bandwagon here) that they figure their average user is not so au fait with a computer and prefer a dedicated button for each feature (for the record Porsche seem to have the same philosophy).
Aside from this, the 7 speed auto was horrendous - even in sport mode it was very lazy and the kick down would not hold a gear if you subsequently lifted off which was very annoying (I like to stab the kickdown for engine braking without having to use the paddles).
So on these 2 points Audi wins hands down - and I can't think of anything I preferred about the Merc over the Audi...