The crucial difference is simply that there are far more 5.1 soundtracks out there than 6.1 soundtracks (no such thing as a 7.1 soundtrack currently available to consumers), and that going with a 7.1 speaker setup requires more space and more funds. Whether or not you choose to go with a 5.1 or 7.1 speaker setup depends on your room setup and your budget.

If your sofa is up against the backwall, forget about 6.1 or 7.1. Stick with a 5.1 setup and optimize your setup for that configuration.

If you have space behind the listening position, then going with the two extra back surround speakers will give you a bit more backfill in the rear soundfield. The purpose of the back surrounds is to provide a little extra surround envelopment. In general though, you should focus on getting the 5.1 configuration done right first, and then figure out whether the extra back surround speakers will provide any benefit.

Since most music sources are two-channel, you only need to worry about the mains and maybe the subwoofer when listening to CDs. Most movies coming out on DVD are 5.1, with older titles encoded in 2.0, and only a few of them prepared in 6.1 (DTS ES) or matrixed 5.1 (DD EX).