how close do you plan on sitting? you should get a rough idea of the size which is suggested from some charts such as those i linked above.
as a gauge i dont have perfect eyesight and prefer to sit about 4ft from my 40" 1080p tv and that is a comfortable distance. ideally i would sit back another 2 feet (which i do for movies) but to see text and other small details on the screen i need to sit closer. being this close i cant concentrate on the whole screen at once though which is why i move back for movies. since i dont have a moveable desk i just deal with being close for games though 2 ft further away would be better (unless reading text in which case where i am now works good). one benefit of being close is that everything is immersive though.
we all have our own preferences though and there is no right or wrong choice.
if you want to save $400 and go with a 55" then go for it. you might need to sit just a bit closer but unless that is a problem for you then this certainly is a viable solution as well.
if you sit a bit farther back and cannot move your desk or seat closer then i'd go larger since i've found having things a bit bigger really helps. of course how good your eyes are has alot to do with this!