Help me choose a bike?

I ride on average 20 miles/day (around). So I'm looking for a more or less 19+ aluminum frame mtb that's light. Also, I don't really worry about the components of the bike because I only use the top gear, sometimes the 2nd (the front gear shifter is completely useless to me).
Right now, I'm having trouble distinguishing between a walmart bike and a bike world bike. Other than the components such as the gear shifters, aren't they the same? I mean the frame and the gears are made of the same materials and everything. Why is one so much more expensive than the other one?

So Basically, neglecting the gear shifters, what are some other reasons why the expensive bike is better?

Well, to help you out, a specialized or giant aluminum frame can be welded alot stronger, be more or less rigid depending on your preference, and has been tested by pretty much every metric known to engineers, to guarantee the fastest wind tunnel testing. How can u believe a wal-mart bike will do this for you? And also, if your not competing in races, it really doesnt make a huge difference. I prefer a stiffer heavier bike for long rides because sometimes i hit pavement and I dont want a sore butt all week long. The heavier bike will absorb more. Since you only use the top gear, I would say components in YOUR case arent a huge deal but handle bars are. They dictate wether or not you go home with a sore back. If you can find a pair that are light enough and comfortable enough for you that arent top of the line then awesome, so far I have not. To put it plainly, your question is very fair because a lot of bikes are marketed toward non professional riders making them think they need so and so's really nice technology for top dollar. Plain and simple, just get a used specailized or giant for a couple hundred dollars thats a few years old. Winter and early spring their are easy deals of lazy riders giving it up. With either specialized or giant and their are others, but those two to me you cant go wrong and you dont have to worry about your budget cause they have models for budgets and needs.

Richard has given you some good details. To add to that, check out Todd Molloy's site on mountain biking as well. It should help you choose the right bike based on your specific criteria. As far as I know, there's more than one reason why you choose a certain bike over the other.