I think the FX series are great bikes. I don't know much about different models in the lineup, so I can't advise you on that, but I can share some personal experience. I started my cycling transformation in early 2009 on a Trek 7.2 FX at about 345 lbs., and I still have it and ride it regularly. It's my commuter bike now, and it sees a lot of rough miles during cycling season. It has held up great, and the only minor issue I've had with it is a creaking crank, so I have to tighten the crank bolts and service the bottom bracket occasionally, but it's a very minor issue.
I'd recommend the FX and not the mountain bike if you're going to be riding mostly on paved paths, roads, trails, etc. I wouldn't opt for the mountain bike unless you're planning to off-road the majority of the time. I still ride my mountain bike, and I find its inefficiency on hard surfaces to be a little discouraging. But, I love it on the trails.