Although guitar & bass look very similar, they are pretty different instruments... it's relatively easier to change from guitar to bass than changing from bass to guitar, but being able to play bass masters stuff like 'billy sheehan' & 'jon myung' definately will take a lot of time & practice.

As I said they are quite different, my advice is to see which instrument you think you'll be better at if you play it, make a decision and stick with it.