The 700 has a powered subwoofer while the 600 does not, making the 700 sub have significantly more punch and also allows for eventual replacement of the sub. This is a huge advantage if you ever think you will be upgrading your components and keeping the receiver as the receiver in both sets is a pretty solid 5.1 receiver, though they don't have too many fancy features, they will do the job quite well for some time.
The issue with upgrade ability in the 600 system is that it does not have a subwoofer output at line level makes upgrading the sub incredibly tough to replace, as most have line in and L/R amp level inputs. However the speaker replacement would be the same for both. Also the 700 has slightly better rear speakers, but its not that significant. Personally I would go with the 700 for the long term.