I will add that ISA has gotten stricter on what they accept for CEUs. They recently did not accept: ASLA National Conference, PLA study or exam, AICP study or exam, Certified Nursery Professional study or exam. I am waiting to see if they accept SITES AP and CPTED certification. I am currently of the opinion they only accept CEUs purchased from ISA. I will update if I discover this not to be the case. Also, their review of CEUS takes weeks, requires you complete one of their detailed CEU forms, requires a detailed agenda, and proof of attendance. It's a cool credential to have, it has helped me to get a job, and the courses they provide do get into technical details, which is nice. However, they are currently rather frustrating to deal with.
SITES AP is also not accepted. I just re-upped my membership and read many articles and completed the quizzes. I like learning, especially about trees, but their refusal to accept outside experience is frustrating and wrong-headed. Heads up to anyone seeking this credential.
I took and passed the exam a few months ago. Similar to what Christa said, initially, they denied my application to take it, but I reached out and actually talked to someone and they deemed my previous experience acceptable. The only thing I studied was the guide they sell on their website. I think if you read through that book a few times and have a solid LA background, you will be OK. As for credentials, I know they include a learning quiz in each Arborist News publication that I think is worth 1 CEU, which is an easy way to earn some of the 30 you need every 2 years.