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.
since both sets have great PQ and are highly recommended, go for the bigger one
Samsung UN46EH6030; Yamaha HTR-3066, SVS Prime Bookshelf Mains and Prime Center, Sony Core Bookshelf (SS-CS5) Side Surrounds, Rythmik LV12R; PS4, Xbox One, Xfinity X1 (CI CXD01ANI)
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.