You don't buy a bike as much as you buy a bike shop, it's like buying a car.... You should really visit both shops, if the staff and customers point and snicker, then your in the wrong shop. If you mention your weight and the guy looks like this-> or this->, then your in the wrong shop. If the sales person simply points at front suspension mountain bikes, without asking about what style of riding you want to do, your in the wrong shop.

If the sales person, greets you properly, asks what kind of riding you want to do, and then what kind of budget you have, your in the right shop. If you mention weight, and they say the only issue is probably going to be wheels, then your in the right shop. Most bicycle brands you find in bike shops are pretty similar, at a similar price point. Frames and components are going to be of similar quality, at the same price. Where there are differences is that some companies may have a slightly different "fit" at the same size. You need to try different bikes, until you find one that feels right to you. It's hard to describe, but the bike will kinda whisper to you, lets go ride a million miles together. See, the idea is, that a bike that fits right and feels right, you will want to ride, a bike that doesn't fit right, and doesn't feel right you will not want to ride.