Good morning!
As you might expect, the answer is, "it depends."
I've gone a few iterations on the speaker configuration in my living/HT room. We started off with 5.1 based on a pair of M3's (bookshelf) on stands as the mains - left and right. I did this to try to minimize the 'visual impact' and please the significant other. They sounded very nice, but something was missing. So, I added a subwoofer and that helped a lot. But after some time we (well, I) decided the bookshelf speakers on the stands looked a little like peanuts in an ocean against the entertainment center and I felt there was a 'hole' in the base response - between the lower-end of the M3's and the upper end of the sub. So, I got a pair of M60's (floor-standing). Now everybody's happy - excellent, continuous frequence response and a nice appearance with the furniture.
I guess the simplistic answer to your question is, if you're going to put them on bookshelves, get bookshelf speakers. If they go on the floor, get floorstanding speakers. Bigger speakers mean deeper bass.
-Brian
