Two different needs are being addressed. Size is usually more important. Even up close, a 55" screen gives a larger representation of the source material. I don't think it's possible to compare size to features.
While I am a 60GT50 owner, I too would go with the 55ST50 if having to choose between a 50 inch and a 55 inch Panel. I really think you will be quite happy with the 55ST50 with the major difference being cosmetic between the 2.