The s4 and s5 both use 300 series carbon. It should be exactly the same frame, just painted different and with different components. I rode an s4 - it's fine, but it's not amazing. I thought it was a better balanced ride than other models at the same price point, but it's nothing I would gush about.

The sl5 and the sl6 both use 500 series carbon, again it should be exactly the same frame for both with just different colors. I rode the sl6 - and it's kind of amazing. It's like riding a cloud, but without feeling completely disconnected. (I've ridden like 15 different bikes this summer, and the Emonda was the only one that stood out for excellent vibration reduction while also having responsive handling, imo - the giant defy I liked the handling more, but not nearly as good of vibration reduction).

I'd definitely pay the extra $540 for the sl5 with the nicer frame if it was me rather than the s5.