Everything else being equal except for the resolution, I'd also suggest the 50". I would recommend going to see those TVs in person, though, because the 42PZ77U is this year's model and has the anti-glare filter. I don't think that the 50PX75U has that (although it might have some anti-reflective coating).
BTW, I'm no expert on these things and haven't seen them in person, yet. I have been doing some research on them, though, and just wanted to mention that.