It really depends on where you plan to take it. For me since I travel a lot, a backpack is very handy (carry-on only). I also live near the beach, so a case is very handy. I ended up purchasing one of each for that reason.
If I didn't travel much, then I would go with a case because I can keep the propellers on. I am tired of taking them on and off when I have to put the P3 in the backpack.