If this is a Unixy system, definitely go with fork()
The bulk of the interpreter will remain shared between children, saving memory. I don't know the threading implementation well enough to know if that holds there. Also the fact that these are separate processes is a big hint to the OS that it doesn't even have to think about concurrency issues. Some versions of *nix actively try to make it more likely that threads will stay on one processor to minimize locking/unlocking costs. Processes are more likely to be migrated freely to balance the load.
Plus fork() has been around for longer, and so has better guarantees of stability.
Cheers,
Ben
