we only have the 64gb version here of the X+, I can't see how you would need any more than that... I haven't even hit 16gb on my One X although I'm constantly file managing on it as I don't like clutter or having too many pictures. I suppose for some the lack of 4G on the X+ can be a problem and I can't understand why they launched it without, other than that it looks great.
I'm not a fan of the GS3, I don't like the cheap looking build, screen or touchwiz but my wife swears by hers and says it's the best phone she's ever had... I don't think Android begins or ends with it so I always tell people to look around first although if you did get it I'm sure you would love it.. I would rather have a Note 2 though if I was going to get a samsung.