If there is a significant other, how does she feel about this? If there is one, definitely get her input, as these TVs are pretty large and definitely affect the look and design of a room, especially when you are talking about a 58" TV.
If I were you, I would do what I did and tape together a bunch of printer paper to the size of both TVs and put it up on your wall. You'll be surprised by the size of the TVs in your own house. I used it to justify breaking over the 50" mark.
As far as size goes, you are going to get differing opinions here. Some say bigger is always better and some are more practical. My opinion would be based on the room function. Would this reside in a basement/game room/ent room/etc?...or is it going to reside in the traditional family/living room? If it's the former, then going larger is never a bad thing. I had a 100" projector screen in my home theater and can attest to the benefits of going large. If it's a living room, then 50" would be ample at 9' viewing distance. I view my PX80 from 12-15 feet in my family room and I am satisfied w/ the size of the picture. Similarly, my wife is satisfied w/ the size and it's affect on the room. If we had tried to go 58" in that room, she would not have been happy with how it consumes the room.
Something to consider also is the wallet. If you are looking at a 720P 50" plasma versus the 1080P, then I assume that you are conscious of cost. If that's the case, you cannot go wrong w/ the PX80 for as low as $800 right now. Then, you can always upgrade later when the economy is a bit more stable.
Good luck.