The performance difference CPU wise between the two for a smaller server wouldn't be too noticeable. Overall, having a higher tickrate does increase server performance (hit registration), but being able to say conclusively you can tell the difference without actually being told it's 100 tick is a bit out there. Though, the standard for CS:S is 100 tick and there wouldn't be any issues not running it as such.
@Kraze^NFo> Juski has a very valid point
@Juski> Got my new signature, thanks!
@Kraze^NFo> Out of context!
@Juski> Doesn't matter!
@Juski> You said I had a valid point! You can't take it back now! It's out there!