WORKER,
Go w/ the dirt bike. I've been riding for years and still love it. Warning, it gets in your blood. Some outgrow it, some don't. I didn't.
I've had every kind from the big 4. I used to fix em up and sell them. This is what I've found:
Honda: Great bike. Midrange punch like Tyson. Very light in front end. I actually raised my rear suspension, to avoid this. With Honda you will pay, but it's worth it. Reliable bike. Do the preventive maintanace.
Yamaha: Great too, very fast. Reliable bike. YZ's are like CR's, your'e gonna pay. I suggest buying used.
Suzuki: I'm a tall guy so the ergs on a RM were a little cramped for me. But they shine in other areas. For example, if you don't feel like shifting gears, don't. No rev limiter. Unlike a CR or YZ, (time you hit the powerband, they are begging for the next gear)
the RM will just scream, how I didn't blow mine up is still beyond me.
Kawasucky: Nice bike, when they are running. My KX was a plug eating monster. This was very annoying to me. Particulaly if we'd stop to rest, shoot the bull, and the fvcking bike fouled a plug. I had to push that bytch back. I wasn't amused. Plus you'll take grief from your buds.
Safety: Wear a full face hemet and goggles, you can't ride w/ others w/out goggles, unless you like hitting trees. You'll see. Wear MX gloves, a rock from the bike in front of you colliding with your hand is not a pleasant experience. Get MX boots, no they won't break in, remember they are for riding, not walking. These aforementioned things I highly reccomend. I have been saved by these items more than once.
I never took any damn class, I learn my lessons the hard way, so you make the call on that one. I actually enjoyed some of the crashes that I was able to walk away from quickly. You'll be more worried about the bike than you, that is if you get bit by the bug. Warning: DBing is cool, but your knees will suffer. Getting sore and arm pump is routine, you'll work alot harder than you think DBing. The dirt bike world is a cool family. Test it out. Hope this helps.
--Crowes