They don't need to get out of UVB light, it doesn't produce heat and if you have a large enough cage and one with enough foliage, I wouldn't be worried about them escaping it. They need as much UVB exposure if they live inside as possible, screens block UVB rays a lot, and artificial UVB is nowhere near as good as natural UVB. So I would get the larger light and expose them to as much as you could. Coils still need to be replaced every 6 months so it's quite possible that your next bulb could possibly be defective. I also do not like exo-terra UVB bulbs because they aren't nearly as strong of an UVB output as arcadia or even zoomed. Zoomed bulbs you also tend to have less eye troubles with. Most exo-terra lighting products are crud, their basking light in my experience burn out very quickly, their hoods are more likely to be faulty, and their UVB output is crud and the bulb itself is more dangerous than most compacts. Anyway it's your choice, but if it's a money issue... I would have never gotten a chameleon in the first place.
