Yeah it's nice. Always on is a nice bonus and having the bigger screen estate on the smaller watch is good.
But I guess with any sort of purchase with Apple it can definitely be a case of "do I need it? No-probably not, but do I want it?". I went from S0 to S3 and was quite happy keeping the S0 as long as I did.
Just so happens that the wife's S3 was giving her grief, I fancied getting the S7 which meant I could then pass the S4 to her so it worked out quite nicely.
So in short terms, if your S4 is fine and you're not that fussed about the S7 then no harm in waiting around for the S8.