Well, obviously, the more power the better. One thing to consider is the question: do you need 7 channels or 5? The 7100 is a 7-channel amp, and the 755 is a 5-channel.

Check the specs on your speakers for the sensitivity rating. This is an indication of how much power you need. The higher the number, the less demanding the speakers are on your amplifier. A sensitivity of, say, 91dB is very gentle and won't require much power, whereas a sensitivity of, say, 83dB would be extremely power-hungry and you'll need all the amp you can get.

Jeff