Physical size is definitely a bigger concern, nothing can beat the Steam Machine. But I do prefer the long and thin design like the PlayStation series has always used. It's easier to fit that on the shelves inside a TV stand that were designed to hold cable boxes and DVD players or it can fit upright behind a TV. I'm not sure if I actually have room to put a Steam Machine behind my TV without it overhanging the edge a bit.
As for loudness, that's a bit more subjective. It depends on how far away you are from the PC and how loud you play sound or if you're using a headset. Just totally based on my experience, games that are very quiet are usually the low-resource indie games that wouldn't be pushing my GPU to max fan speed anyway. The games I have that really push my GPU tend to have pretty loud sound design.