You didn't specifically mention your goals, so it's a little hard to know what exactly you're looking for, but here's some info that might help you.
1. Nat and Vince at Under the jungle are the two best dive instructors I have ever met, and I've been a professional educator for most of my life, so I'm good at evaluating instruction. The culture and standards at that shop are fantastic. And they're pretty GUE-friendly, there are lots of divers at the shop who come from GUE backgrounds.
2. For cave diving in Mexico, sidemount is overall better IMO, and I spent a few years diving BM in those caves before I switched, so I'm very familiar with both. I did intro and full cave courses at UTJ in backmount. If your goal is to cave dive in Mexico regularly, it's worth going for SM at some point, but there's nothing wrong with doing your courses in BM. The reasons I think SM is better in MX are several; most people are diving SM, so equipment matching with buddies is a bit easier, the topography lends itself well to SM, most restrictions are a little easier in SM and there are lots of SM-only passages where it's just too tight vertically for BM. It's also much easier with entries/exits in some of the tiny, rocky, slippery-slope cenote entrances. It's just overall physically easier and more flexible in the caves.
3. I think Mexico has some advantages for cave training. The caves are generally very shallow, so intro dives can be pretty decent. There's very low flow, which makes most things easier to learn, but also presents some extra challenges, particularly in terms of dealing with silt outs and some navigation things. Ultimately if your goal is to dive in FL, that's where you should train, but as someone mentioned there is no reason why you couldn't take some classes in both places.
One other thing, Mexico is generally a really fun place to hang out (for me), and it's cheap. Some people would rather not be in a country where the language is different, the food is different, etc... but I do. That's a personal preference.
I would send Nat an email and ask her all your questions. The cost for training there is very reasonable considering the quality and effort that goes into it. How much a course ends up costing depends on how many days it takes you to finish, and no good instructor can tell you that without first evaluating your dive skills.
R I’d go with cave 1 in MX or FL depending on where you’ll be doing most of your cave diving, get 50-100 Cave 1 level backmount dives in, and see where you’re at.
