Well, I've done it both ways. I had to fabricate a two servo elevator installation in a Goldberg Matrix when that bizarre through-the-fuselage bellcrank arrangement it comes with would not smooth up.

YMMV, but I find it difficult to see how building a simple bellcrank on a platform is more involved than cutting into the bottom of the fuselage, adding holes in the side of the fuselage, building sufficiently sturdy servo mounts, routing the servo wires, and closing it all back up with care so it looks halfway decent.

Good luck with whatever you decide to do.