With brickfilms, the range of motion on minifigures is so small that there is not much signifigant difference between 15 and 24 fps, as far as I can tell.
Don't do 48.

EDIT: Also, good luck!!

I'm gonna be dead honest here, and not too technical.

I don't know you, but many brickfilmers (like me) often get inspired to to a great idea that's in their minds, "oh it's gonna be awesome, it'll be full-length, at 24fps" and they often get about as far as editing 2-5 minutes of it before they start procrastinating more. So if you're really going to finish this, then go for 15fps. Not much of a visual difference between the two, and most likely, attempting 24fps if you're not someone like Mobdeli, is gonna look sucky.

I'm sorry, don't want to burst your inspirational bubble, here, but this has happened to me. I'm still finishing something from 2007, and my animation, editing and effects have greatly improved over time, so having to "downgrade my abilities" for that film is a hard experience in and of itself. Until now, though, I've only cancelled one project, which is good, but that was because I thought hard before attempting any new project.

Brickfilmer a decade ago, now looking to relive the glory days