I'd say the critical question really isn't around speed of an interpreter, since if you care about speed you're using something like a JIT compilation model to actual machine code anyway. A more important question, if you want the VM to become more widely useful, is to make the instruction set easy to compile to.

A big advantage of a stack-based architecture (in my experience) is that it's simpler to write a compiler for because you don't have to worry about register allocation. My compiler writing experience is a bit limited (I've done some work targeting the JVM, and some very limited MIPS assembly coding), but I've found that not having to track register state is a very helpful simplification.