I was in the I-wanna-go-wider-than-90 club a while back, and scored a pretty good deal on a Super-Angulon 75mm f/5.6. It's darn nice, and it's
very wide; I think any wider would be just too wide.

I don't have the center filter, and should probably pick one up at some point, as there is definite falloff. I've got a 90 (also a S-A f/5.6), and that's sometimes iffy on the falloff, but the 75 pretty clearly needs falloff help. I'd imagine the Super-Symmar probably would need the same help, and I'm sure the Grandagon would as well.
I have heard that the difference between 65 and 75 is not enough to make you want both, and 75 and 80 are very close too, so I'd suggest trying both, to see just how wide you want to go, before you make the plunge. And your eye may just like one focal length over the other.
Will