I think this is actually a very good question, because some portions of the classes are quite similar, and people often have an erroneous conception of what Fundies is, and how GUE goes about teaching it. As someone who has taken GUE-F, Cavern, Intro and GUE Cave 1, here's my take on it:
First off, not everybody lives where there are caverns. GUE-F can be taught anywhere there's water to dive in. If there isn't a local GUE instructor, there's always someone who will come in to teach a class, if you can get enough students together. If you have to travel for either class, that will change the equation a little.
Second, GUE-F focuses on skills that are applicable in any diving environment. You won't spend time running line or following it blind, because those are specific overhead environment skills. You WILL spend time practicing precise buoyancy, perfecting horizontal trim, learning non-silting propulsion skills, and doing emergency drills. All of those skills are useful in any diving environment. The only thing in GUE-F that isn't is bag shooting, and that is only NOT useful in the overhead.
Third, people get WAY too worked up about "practicing for the class". GUE-F is a class that introduces you to a lot of new things and a completely new standard for diving skill (at least for most of us). PASSING the class is only necessary if you want to take a more advanced GUE course. Taking it and getting a provisional will still leave you with a ton of information and some new skills, even if they're kind of embryonic, and a good idea of what and how to practice to continue to develop them. In my experience, the skills I got from Fundies increased the FUN I have with every single dive I do, in ANY environment, and even with ANY buddies.
DIR diving tends to attract people who want to be really good at something, and those people can get pretty intense and overfocused at times (am I describing myself, maybe?) You don't have to spend all your time in a quarry doing ascent drills to take or benefit from GUE-F. Some directly applied practice definitely pays off, though; at least for me, some of these skills have been very hard to master to the standards of the agency, and have required some real work. But a lot of the practice can be done during any fun dive . . . Working on hovering, using your back kick to maintain positioning, practicing good team communication. These things all add to the quality of every dive you do, and you don't have to dedicate any specific dive to working on them.
My understanding is that a cavern class will spend time working on buoyancy, trim, and non-silting propulsion. Mine didn't, because it didn't have to; we had all those things down, and could spend our time in the class working on overhead-specific things like line running and following, blind exits and the like. My personal feeling is that, if you are interested in diving in overhead environments, it makes sense to master the skills you can in open water, before you travel for the overhead training, so that you can maximize your very environment-specific learning. I know for sure that we got to do better dives in our Intro to Cave class (which immediately followed cavern) simply because we had our basic skills nailed before we got there.
That's my take on it. I think anybody considering overhead training should do GUE-F at home, or close to home, and spend time practicing before they travel for their cave training. It certainly worked beautifully for me.
And fisher, unless you have been diving with GUE or cave divers and getting some mentoring, I will almost guarantee you that the class will be difficult, and that you may not pass it. It's okay! A lot of us didn't pass the class the first time; I took the whole six month grace period to get my rec pass (which simply means single tank). But the improvement in my diving and in my FUN started long before the six months were up.
