Striclty for HAM stuff, 77 is great for VHF/UHF.
I own the 77 and the 815. 77 will definately give you more reception and reach just based on it's size.
In the area that I live, I prefer the 815 because of it's size, and it gets me all the repeaters I need that I could also hit with the 77.
If you plan on using the antenna for scanning also, again 77 is awesome on VHF/UHF, but loses out on 800 over the 815.
On a side note:
I have been searching for an antenna that will do VHF all the way upto 800 for scanning, and I've gone through several antennas, and I'm back to the stock Uniden duckie that came with my BCD396 because it gets me everything I want better than any aftermarket so far. I am considering buying the Diamond SHR805 to use for local stuff. Anyone have experience with this little guy? Only 1.75" tall !!!