The numbers 5.1 and 7.1 denote the number of speaker channels and the subwoofers used in the music system. A 5.1 channel surround system comes with 6 channels in total where 5 are speakers and the 0.1 denotes the subwoofer.  7.1, on the other hand, is an 8 channel system that comes with 7 speakers and a single subwoofer. When compared purely on channel-wise, you can choose the 7.1 surround system. But again, you need to check with the output power of each of the individual speakers. If the 5.1 channel speakers have higher output power than all the speakers of the 7.1 system combined, the 5.1 channel system is right better than the 7.1 system.