SheepShaver is kind of crashy for me. Basilisk II runs great though.
The difference is that SheepShaver emulates a PPC while Basilisk II emulates a 68K. This means when you run GameMaker on SheepShaver on an Intel Mac, it has to convert 68K to PPC to Intel which makes it slow. Basilisk II would go directly from 68K to Intel, and therefore be faster.
The maximum OS for Basilisk II is OS 8.1, but you can download System 7.5.3 legally for free, from Apple, for the true GM/System 7 experience.