I may be wrong, but I believe it actually depends on what state you are in.
Being a vet would definitely help your credibility, but honestly that is a TON of schooling! It is my understanding that in order to be an equine chiropractor you need to have learned human chiropractic first as well. Again, just going off what I believe to be true here, so double check!
I would honestly guess, you probably are better off choosing one or the other, just because I doubt the additional credibility of being a vet would make up the cost and time of all the additional schooling. As a trainer, once I find a chiropractor who is good, I continue to use him/her and refer them to all of my clients. I think if you are good and get results, people will naturally flock to you!