I have had both. The S2 has better glass, is smaller and touch lighter. It is a "niche" type of product for sure.
The CS1 is a nice scope, trim on size for a 65mm and has decent glass that I believed outperformed my Gen 1 Razor 65mm that I compared it to. Overall, the CS1 is probably a more well rounded scope for most uses.
I kept my S2 and also my S1.2S 80mm spotter. Figured if I really needed a bigger scope, I needed a bigger scope.
I prefer 15x binoculars to my S2 so I am likely going to sell it sometime this year. When I was using the larger Meopta 15x56 binos, the weight penalty was enough that I would sometimes take the S2 instead. But now with my 15x50 GPO Passion HD bino, I will likely use it more often than the S2 spotter.
If you are interested in an S2, let me know and I will sell you mine for a good price.
Good luck deciding. Lots of good options out there.
FH