1. you don't need any documents from the state board to take your regional exam. it is through the dental school you are going to. some dental school offer both WREB/ADEX, some school offer Adex only, either CBCA or CITA, some offer only SRTA. so if you want to take an exam your school does not offer to go to the state that only accepts that particular exam, you have to fly to a different dental school to take it. this is pretty nightmare-ish.
2. as most people only take either or and never take both at the same time so i cant tell you the difference on the experience. it is mostly depending on the states you want to allocate to after graduation. the big difference for WREB and ADEX is before California does not take ADEX but recently I've heard it also takes ADEX.
as far as I can tell, I've heard finding clinical lesions on live patient for ADEX is a bit easier since it does not require the cavity to cross the DEJ, however you have to do one class II and one class III. For WREB, the lesion must definitely cross DEJ and depending on the state, you can do one class II only or have to do both.
I hope my answer helps. if anything, ask again