BEST MOUNTAIN BIKE REVIEWS

I like to bike on Bike paths and on Side streets for fitness. Maybe 50-70 miles a week. I weigh 210, So I figure mountain bikes would be more durable. I usually buy Mountain bikes for 0 or so from places like Walmart or K-mart. That being said I am looking for a better Quality bike, Somewhere between 200-350. I am most concerned about safety and durability....Should I buy a High quality Used bike from E-bay,,,,I high end Model from a Walmart,,,Or should I go to a Bike shop and buy a Entry level Bike???...I wen't to Bert's bikes in Tonawanda NY and they recomended a Trek-820 for 329.99 with free lifetime service.. Sounded pretty good....So I figured I would ask the Smart riders of Yahoo Answers for some Good Advices...Thanks!!!!

I am putting road tires on this bike, As I plan on riding on pavement and bike paths exclusively...I figured a 16 is better for control + stability (As in off road riding)...While a 18 inch would be a little better for the path...I plan on only road riding, So I think I should go for a 18.....Any advices ????