5.1 vs 7.1

Kal gets to the heart of it when he asks how much 7.1 material you have. If you largely watch Direct or cable you are only getting 5.1 anyway.

For me your question is room related. Strictly speaking, 5.1 places two speakers about 2/3rd way back the room on the sidewalls firing across the audience at each other. 7.1 adds a pair of speakers on the back wall firing across the audience back towards the screen. (draw this out, its simpler to see it then write it)

7.1 gives the sound designer the added ability to work back to front as well as side to side.

If your room is long so that your seating position is at least a few feet off the back wall and a few feet in from the side walls, chances are that the 7.1 would be a nice upgrade especially if you have a very big screen and are after a real theatrical experience.

If like most of us you are struggling to get some distance from the side speakers or are already mounting your 5.1 speakers on the back wall, it's probably not going to be much of an improvement.

The smoothness you hear is the result of the extra speakers - effectively as an effect through some part of the room, each "move" is a smaller increment so the change of speaker location is not as pronounced.

Very often those speakers are not used for panning effects but to provide extra ambience - you are inside a sub - you don't really notice it but it colors your impression. Just as a subwoofer does when its not thumping away doing dinosaurs and bombs.