I'd first suggest driving both.. I will tell you - I have an A7 with APR tune and AWE exhaust About 18 mos ago i test drove an identically optioned and colored pre-facelift S7 at my dealer. It was a fairly easy decision for me not to make the switch to that car as it did not really excite my any more than my car, and in fact was kind of a let down because i really wanted to get excited about it. But here's the caveat...i was already tuned to around 450HP and had a nice exhaust on my car and i was test driving a STOCK, pre-facelift S7 with a much blander, much quieter exhaust.
Had i test drove a car with sports exhaust and perhaps even tuned - i think my time in the driver seat would have put a bigger smile on my face. I do wish i had the S-sport seats in my car and that was the biggest differentiator i felt during my drive. Having said all that, an S7 or RS7 is def a possibility for my next upgrade now that i've gotten a lot more usage out of my investments in the A7 and will be ready to start modding another one!

Sorry if that confused you further..but as you can see it's a very subjective decision with a lot of variables involved for each person like what you're coming from, timing, investment, etc. In a perfect world, we'd all just go buy an RS7 and skip all the questioning and steps in between. lol