first just do the comparison on price vs. performance. I am not going to look this up for you. You can go see that 7770 is on the toms best bang for the buck chart recommendation currently.
Now onto comparisons for only very special builds:
7750 doesn't require extra power supply plug. You can also choose a few models that are fanless if you need to do an absolutely silent build, or home theater PC build.
7770 requires a powersupply plug , but still is very low power compared to other cards. It gives more performance for a relatively slight price increase.
I think there is only 1 sapphire model that is fanless. So most have fans, but if you choose wisely, there are many fanned 7770s available that still are very quiet.