Tried them both yesterday and the S4 felt more refined and better to hold on to. They are VERY similar next to each other and about the same height even though the S4 has a slightly bigger screen. Both felt great and upgrade my wife to an S3, she loved it.I was playing with a co worker's S4. It feels COMPLETELY different in your hand than the S3, much more comfortable IMO.
Looks like they took more cues from the Note 2 which IMO is a great thing.