I voted for gila monster, but I think the biggest question is going to be what's going to be the easiest for you to deal with the housing requirements. From what I've read, gilas are relatively placid animals so require a very small amount of space considering their size. On the opposite side of the spectrum, a caiman lizard would require an enclosure about the size of a large walk-in closet. I think especially if you're not planning on staying where you're currently living at for many years, go with the gila whose enclosure will be much easier to transfer to a new home, and wait to get the caiman lizard until you're in a more permanent housing situation.
EDIT: And where the hell can you get your hands on a gila for $500? All the ones I've seen are in the 1700-2000 range...