I have a GTI and a Tiguan, which I know isn't a Q5 (Golf platform vs A4). I've looked at the Q5's and they're nice. Bigger interior space than the Tiguan for sure.
I find the Tiguan and my GTI drive very similar, however, the Tiguan is really let down by not having a DSG in my opinion. With that said, the Q5 is bigger, so you can expect there to be a sacrifice in driving feel. The Q5's perform really nice, but its a bigger car of course.
The Q5 will be a total step up in fit/finish, but will also be a set up in cost of ownership. I've read the Q5 is less reliable than the Tiguan, which is a bad thing. My Tiguan has a bit over 34K km's and I've just had to replace two injectors due to carbon.