4k games, Free games, Discounts, 5.1 sound support. Just that. Also, non-pro will only be available next year
@robf Not sure if you've seen yet, but here's description with regards to Pro membership from the help page
What's Stadia Pro?
With Stadia Pro you can play in up to 4K at 60 FPS with 5.1 surround sound and get discounts on game purchases. It also includes access to an ever-expanding library of games. You’ll have the best Stadia experience with Stadia Pro.
You can also check the FAQ which contain more info with regards to the Pro membership
Also I think it's worth mentioning that for now you can only play games through Stadia if you have Pro membership, and the only way to get pro membership now is to get the Founder/Premiere Edition. The base version will be released next year.