Most people when they go looking at bicycles tend to focus their attention on the most recognizable and marketed items. (mainly Shimano drivetrain components). Please don't do this. I'll tell you why. You, at 350 pounds, will benefit MUCH more from having a stout, quality wheelset (and crankset too) than you would from having a really nice rear derailleur. All through high school and undergrad, I worked in a bike/ski shop (mainly to support my habit). And all those years I was on a personal quest to get people to see the wisdom of quality bearing sets vs. high end rear derailleurs. Break a shifter, 50 bucks. Break or prematurely wear out a wheel or rim, at least 150 bucks (for a cheap rebuild). I can tell you at 300 pounds, you need the best hubs and rims you can get your hands on. Here's what I would do: see if you can get the nice guy at your local shop to trade out the stock wheelset, for something nicer and more heavy-duty (unless the ride you are looking at has good hubs and hoops to begin with).

Look around. Sun-Ringle makes some nice wheelsets at a good price. Chris King makes the ultimate (but could be out of your price range.) DT/Hugi are probably the most durable hubs on the planet. I have them on a FS tandem I built up, no problems. Some may say that you will need a 36 spoke set-up. I would disagree with this in theory, but if you are very aggressive, it may be worthwhile (my tandem has done fine with a 32 spoke DT 14/17 revoloution lacing[if you know what those are])

For your weight, disc brakes would be the way to go. But only if they are laced to disc specific rims. The cross-section (i.e. lack of a sidewall) of these types give them increased strength to weight ratio. And discs, hydraulic or cable actuated, are really much more powerful.

The brand of bike is less of an issue for me. Before Giant started making their own brand back in the late 80s-early 90s, they made frames for many large manufacturers. So they know what they are doing frame-wise. Their new carbon frames are particularly nice albeit expensive.

Wow, I haven't talked shop in WAYYYYY too long. Thank you for breaking my rut.

Any other questions, feel free to ask. This place is rife with MTBers, so I'm sure good advice will never be far away.

Take home point: good wheels!

Have fun,
Greg