The question is, what new features would you want?
I was thinking about this, and reckon that the split screen thing I could genuinely see uses for. However, I believe it only works with a limited range of apps, which makes it a lot less useful (I have my own app preferences, and may not want to be forced to work with Samsung's versions). The "smart stay" thing, as long as it works well, would also be useful to me. But both of those are things you can get on the S3 already.
The new stuff with the S4? Maybe there's something I've overlooked, but from what I recall none of it seemed as useful as these. Now in fairness I could also imagine never using BlinkFeed and never putting the One's camera in Zoe mode, so for me possibly the new features in both are a wash. But people's preferences and requirements differ, and if the S4 features appeal to you, go for it.