550B here for me because I wanted maximum flexibility and prefer manual indexing. With auto-indexing, if you get part way through a stroke and something goes wrong, you can't just back out of it. You have to disassemble, clear and then reassemble. If you are doing pistol calibers, I'd recommend getting a dedicated 450 for each caliber, but that's a whole other story. Dillon all the way. If I lived 25 minutes from them, I'd probably be there daily...