I thought FPSC could do multiplayer but my guess is it's going to be irrelevant as you may have elements which cannot be done in FPSC without a lot of scripting. In that case I'd go DBP anyway.
Hard to say without you sharing some of the requirements of your game.
Seriously though, unless you've come up with something revolutionary (which would be difficult for a multiplayer FPS) I doubt anyone here will steal your idea. Most of the fun of creating games is in creating your
own game.
Anyway I guess I'd say if you are prepared to invest a lot of time use DBP. It's more flexible and the results
can be much better.