Funny, I'm just reading Bill James book on the HOF for the first time in year; the Tiant/Catfish thing is a decent part of the book. Really good point, Catfish had the 5 20 win seasons and not a whole lot else.
I don't like Schilling, but my gut says him.
Quickly scanning their bbref pages; Mussina has about 55 more wins than Schilling, which is a lot, but "only" 300 more IP. Schilling's era is slightly (slightly I say) more impressive. Schilling has some of the best K/BB numbers ever, and some amazing post season stats. So I think i go him. Both guys are easy HOF to me though