With all the talk of FF vs. APS-C, and what changes and what doesn't, just keep in mind that working distance is the important factor, for a few reasons:
1. Will there be enough room for you to get far enough away to frame things how you want? Shooting in smaller spaces, you might find yourself backed up against a wall from time to time with the 77.
2. Can you communicate easily with your subject, and are they comfortable with how close/far you are. This is maybe a subtle thing, but I shoot a lot of portraits, and good rapport with the subject is critical to getting them relaxed, and getting natural expressions. This distance will be different for different people and styles, but I find most often for how I frame and how I communicate, that 100mm on FF is perfect. When I shoot portraits at 58mm on FF, it feels a little like I am invading space, and I honestly get more relaxed people if I can stand a bit further away. Maybe it's because I'm scary-looking, I don't know.
3. Perspective distortion. For the same size sensor, assuming similar framing, there IS a difference between 50 and 77. It's not a glaring, obvious difference that will scream at everyone, but it's a real difference, and if you shoot a lot of portraits, you'll begin to feel it. I shoot so much at 100+ that when I look at stuff I've shot with my 58 on FF, or my 35 on the K-5, it almost looks fisheye to me now. Again, this is all personal.