I made that jump as well. I had (and have) three AWs and wanted an AWS2, mainly for the brighter screen. What I found was the screen wasn't noticeable brighter to me. I've yet to use the GPS, but thought/think it would/will be handy one day should I forget my phone. I don't run and don't know if or when I'll swim again. There are some areas where the S2 performs faster, but they're few and far between. watchOS 3 works well on all generations. The move satisfied my curiosity. But unless someone needs GPS, is an avid swimmer and/or runner, or just wants the Nike+ (as I do), I couldn't recommend going to an S2, unless money just wasn't a metric at all.
The S1 and S2 will have the edge down the road as wOS become more demanding, but I see no reason to upgrade the rest of my AWs. Maybe when S3 or or S4 rolls out...
If I knew then what I know now, the smart money would be on the AWS1. Same SoC (except for the GPS bit) but 450nits for the screen instead of 1000. Light behaving the way it does, I don't think real world use is anywhere near as dramatic as the stats. It'll work as a driving assistant as long as you have your phone with you, but not on its own. But if you got a good deal on an AW, you might be very happy with it.