While it sounds like the OP already made his decision, I thought I would chime in. I don't own an MkVI GTI or an A4, but I've been looking hard at dumping my MkV GTI for a CPO B7 A4. I love the design of the B7 even more than the B8...I think.
I could be imagining it, or perhaps subconsciously I don't really want to dump my MkV, but when I got back in my GTI after test driving the A4s, I can honestly say that my GTI felt 'tighter' and dare I say even of 'higher quality' than the A4. I was actually happy to get back into my GTI.
I was not overly impressed at all with the interior of the A4 even as it compares to my MkV. This might have something to do with the fact that the A4s I drove were leased and had some noticeable scuffing on the interior trim. The A4 had nowhere near the steering response and road feel as my GTI and was much less communicative.
That said, I still really love the design of the B7, and with a tune, I could still be swayed. But for now, I'm sticking my MkV.