I recently had to make that decision, moving to the Boston area with my family for a year or two. We only brought with us the two pieces of luggage that the airlines allow, so I could not possibly bring all the parts we have. But at the end, I really avoided making difficult decisions (like how many 24t gears to take). I just packed almost all my NXT and Technic pieces as tightly as possible and took them with me. When you pack tightly, the parts don’t take that much space. I left behind only the studded beams (except for a few), and all the non-Technic Lego parts.