I got an A3 when it was in for service and then an A5 sportback when in for some warranty. I didn't specifically request anything apart from it having rear doors so I could fit child seats though. If you don't ask then you don't get, so always worth asking the question!
Previously I've always been given an A3. Last month was offered an A4 or A5 when I got to the dealers. Took the 5 just as something different. Only need to go 12 miles home and back again so not too concerned as to what I'm given.