Probably depends on how good you are.  The object is to finish with everything on the same plane.  How about removing the bulk of the matrial with your sander and finishing with a sharp plane?

Sure, whatever will remove the material will work. They don't have to be perfect, either. I purposely aimed for a tiny gap between the decks and the inboard edges of the sheer clamps, because I knew I couldn't get all the angles perfect, and did not want any gaps to show along the outside of the sheer, between the decks and the side panels. I just made sure I applied enough thickened epoxy in the joint to bridge any gaps on the inside of the boat.

Convenience, for me. By using the plane I get to skip some safety gear. Plane is quieter, shavings easier to clean up than dust (and more difficult to inhale).

Laszlo

I bought my Stanley block plane years ago for cabinet making and rarely used it. But when I got it out to plane my shear clamps, I was amazed how well it worked. I'm with Laszlo, no dust, no mask, no ear protection, no noise(especially if your running a dust vacuum while sanding), no electric bill. And I'd bet its faster as well.  Plus, you can use the bag of shavings for fire kindling.  If the plane is not cutting well, maybe it just needs some sharpening or you're adjustment is a little off.  Dave