That's really 2 different game that you can't compare.
CSS is a FPS and Rust is a survival.
But if you want i would more spend money on Rust than CSS for the fact that Rust is a sand box and can be edited really more than CSS, so this give alot more possibility of gameplay for only 1 game.. Or i gonna just buy CS:GO for the huge change it have from CSS.
As i said, it's a retarded debat cause compare an FPS to a Survival is just impossible, it's a personal like or not.
This debat would exist if you wanted to compare COD to CS then ok but not like that.