wow, looks like alot of us were into bmx and mtb.
I really need to get back to riding too.
my first mtb was a nishiki ariel with the elevated chainstay, bought used. I still have my second mtb a 94 kona explosif. my current ride is my trusty ibis mojo and dean private both hardtails, but to be more with the times I have been eyeing this frame:
this would be stout and burly-good for singlestrack and freeriding.
I do have a soft spot for kona though.
oh yeah, also still have a kz-1 kuwahara and db silverstreak in my garage plus other vintage mtb and bmx stuff.
I'm fond of the Santa Cruz Heckler and Specialized Enduro. Both of these bikes are great all round trail bikes that are built for guys that weigh more than 175. When you narrow down your choices go check out Mtbr.com and find yourself a good deal. There are some sweet bikes on there and some great values.
You are an ideal candidate for a steel framed solid fork mountain bike. Use it for everything including trails. You can probably find one at a garage sale for $200. Once you have ridden a season and know that you are comitted, then fork out the bones for a nice ride. Is there any possibility of riding to work? That builds a mileage base faster than anything else. At 300 pounds regardless of fitness, you will not be a strong climber, so the weight of your bike does not matter. Actually your 39 pound bike does not matter. Just ride that. It's only 14 pounds heavier than a light mountain bike at 25 pounds. You'd be better off getting the 14 pounds off your body, if you know what I mean.
I'm a long time bike commuter and tend to build a new bike every few years, usually out of leftovers and old school parts. Lots of cool stuff out there. Any interest in building a road bike? Heavier guys do great on road bikes built out of old mountain bike frames. We can discuss that idea at Rubithon.