Having the OS installed on the SSD provides far more benefit than just start up times. Every single OS action involving that drive will be faster including paging and the loading of applications.
If you were to use the SSD for your games you'd only potentially see an improvement. It's largely down to how the game has been implemented. The game might start quicker, it may load saved games faster but you're unlikely to get increased framerates or anything.
For this reason I'd use it for the OS. It will have a greater widespread performance improvement for your system.