they're pretty close, I read a report some place (yeah I know) that repti-sun was closer to the natrual sun light or some such, I've also heard repti-glo starts to degrade (the UVB output) sooner than repti-sun so it should be changed more often , but I don't have a UVB metor so I can not confirm that.
if it's the tube style I think either work fine (stay way from Zilla tho, or so I've heard) just be aware the repti-glo may need changed sooner (I think 6 months is recomened vs 9 months for repti-sun, of coarse if you have a UVB meter you can take the guess work out of it and wait till the output starts dropping off)