EE is simply a patched MRI, so it will be faster on strings (known issues with rbx), but rbx has generation GC and JIT which can only improve.

Right now in my limited testing the performance is a wash, but if your app is memory hungry the improved GC is worth it alone. If your app is CPU heavy the JIT becomes your friend. The only way to be sure its to test it (how about running your test suite on it?), but I suspect rbx is going to get very fast as soon as a few big players shift on to it and the sweet spots for the tuning knobs are discovered.

The code is pure ruby, it creates a decent sized trie then walks it once to set data in each node (a sequential forward index, etc).

I'll release the code once my library is ready for public consumption, but the numbers were (in seconds):

rubinius ~ 4.85

jruby 1.5 ~ 5.45

mri 1.8.7 ~ 9.35

ree 1.8.7 ~ 9.35

and Yes, I did a warm up.