I think I was a few weeks ago in the same situation as shamael. Finally, I went for the stereo amp since I thought that I will be listening less to “real” surround sound than stereo.
Since I wanted to use one of the optical inputs of the amp, my setup was first RPi → HDMI → TV → Toslink → Amp. Having the TV on when listening to music started to annoy me and I bought the hifiberry digi sound card to go directly from the Pi to the amp and that I could stream via airplay. In my case I just have to switch then between the inputs on the amp.
@powaq I use shairport for the airport streaming. Here is a good introduction, but I still had to play a lot with the settings.
I would be a bit in doubt if the Hifiberry Amp would be powerful enough, but according to the specs it should be good.
Good luck!