Mountain Bike for a big guy.. i need help!?

i went to a local bike shop and got fitted. I'm 6'2 and around 250lbs. the shop was insisting that the 19 sized bike would suffice. but i found that the seat post was high out and my worry was that in time it might bend because of my weight, like what had happened to my old bike. disheartened, i just decided to go home and try another shop because the people there weren't really accomodating. but before i go to another shop i'd like to get some insights. what do u think is the right size for me? and also, what frame material (alloy, steel, etc.) would suit me best?