I am no FitNesse guy, so please take what I say with a grain of salt. To me it seems what FitNesse is trying to do is to provide a programming language independent environment to specify tests. They use it to have a more visual interface with the programmer. In Spock a Groovy ast transform is used to transform the table into a groovy program.
Since you basically stay in a programming language it is in Spock more easy to realize more complicated test setups. As a result you often seem to have to write fixture code in FitNesse.
I personally don't need a test execution button, I like the direct approach. I like not having to take of even more classes, only to enable testing and I like looking at the code directly. For example I want to just execute my test from the command line, not from a web interface. That is surely possible in FitNesse too, but as a result the whole visual thing FitNesse is trying to give the user is just ballast for me. That's why I would choose Spock over FitNesse.
The advantage of the language agnostic approach is of course, that a lot of test specifications can be used for Java and for .Net. so if that is a requirement for you, you may want to judge different. It usually is not to me.
As for performance, I would not worry too much about that part.
