Hi. The chief difference across the two series is the number of SATA ports available.
The 7600 series has 2 SATA ports for two hard drives
The 7700 series has 4 SATA ports for four hard drives
There are also some other minor differences. The 7600 NVRs typically have 4/1 alarm input / output terminals, whereas the 7700 series typically have 16/4 instead. Similarly, the 7700 series often have an extra USB input - again, minor stuff.
I've also been led to believe that there are hardware differences between the two, and that the 7700 series is a more powerful NVR, but I'm not sure how accurate this is. It's certainly never been something we've noticed, and the specs are extremely similar as you've noticed.