The S4 is a fantastic car, but the 2.0T does have positives too. I like the B7 shape, just as much as the B6. The fuel economy would be better with the 2.0T. And if your into your modding, then the 2.0T has such potential mod-wise. You could quite easily be getting the same power as the S4 V8 with a few upgrades. (But again depends if you want to spend more money doing that, and the mods do hurt resale value.)
But saying that, the B6 is a lovely car. The sound of the V8 is to die for. So its just if you can justify the economy.