I don't compete or do heavy duty riding. I needed a bike to commute to work and hit local stores. Later on, I started doing light trails then got a cheap road bike.
I've originally got a Walmart bike. The reasons I'm not recommending them is:
1) They're cheaply made and one of the parts will break sooner than later. Whether it's a bent rim, broken pedal or whatever.
2) They ride like shit. They shift like shit and the cranks are usually shitty.
3) If you're 6ft + or have long legs, they're too small. That's why I asked you for your height. On the Walmart website, it'll say they're 26" inches. This is the wheel size, not the size of the bike. The actual frame size is important. It's measured from the middle of the crank to where the seat post connects to the frame. I personally need this to be over 19" for the bike to be comfortable.
Why would you not add the extra $100 to your budget and get something decent from a bike shop? What's a $100 in the long run?
If you're not carrying the thing around, you'll be fine with the Giant Boulder even though it has a steel frame. I used it to commute to work for 2 years and part of this commute was on a dirt trail. It's a good bike and I got it for like $250 at a bike shop.