I researched the Fisher, ans several other manufacture's model in that price range. I found the following differences. The frame. There were other semmingly insignifigant differences such as the saddle, grips, bars, stems, etc. However I am concerned that at your weight, I am at 200 myself, that if you are going to ride off road with frequency that the components and wheels will not hold up long. These models of ATB's are intended for recreational riding and rather tame trails. You can ride it of course wherever you please but the durability is the question. The primary components are not intended for this usage. I know that price has a great deal to do with it. I am going to reccomend that you jack up your window another 150 or 200 bucks. Then you can get a signifigant upgrade in componentry that will last alot longer. In specific the wheels shifter's and derailleurs. However you are the one who will be riding this and that is all that matters. If you like it that is all that matters. If you like the Fisher and that is your price range then you might want to look at Raleigh's for that price range you can get better spec's. Also I will say that pound for pound a good Cro-Moly frame is stronger than aluminum. You will also pay less for steel but it is heavier. Not by much maybe 1 to 2 pounds total bike weight.
That is my opinion. Hope this helps.