Theatrical releases are still being handled as 5.1 in the majority of cases, so producing 6.1 or 7.1 mixes involves repeating a lot of effort - as a result, it is still fairly rare to see 6.1/7.1 very often. There have been a few ( this AVS thread specifically mentions at least one 7.1 and one 6.1 mix, out of a sampling of more than sixty titles), but it's likely to remain hit-or-miss for the time being. On the other hand, there are still some very good matrix processing modes that can generate pretty respectable rear surround data on the fly (Pro Logic IIx being the obvious and most common example).

Either way, I would select the speaker layout based on room layout, not on source material availability. If your seating layout isn't conducive to a 7.1 setup, you'd be better off with 5.1 and let the system downmix the rear surround signals into the side surrounds. A 20'x25' room is certainly large enough to support a 7.1 speaker setup in theory, but seating placement is a significant factor. If you have seating directly against the rear wall, that is not going to be a great situation for 7.1. In my 23'x17' space, the couch is 5' or 6' off the back wall (essentially a third of the way across the room's short dimension). My direct-radiating side surround speakers are in line with it on the side walls and are only slightly farther away than the dipole rear surrounds. This arrangement has worked quite nicely for me.