General riding tips:
www.leelikesbikes.com
Steel versus aluminum: Aluminum is the value choice. Good steel frames tend to cost a bit more. Either is fine.
26er versus 29er: If you are really short, go 26er. If you're really tall, definitely go 29er. If you're in between, you can go either way and it's down to preference. I ride both, but prefer 29ers when I'm aiming to cover some distance.
That $150 bike: Don't buy it. You're not even in the game with that "bike".
FWIW, I am not against all x-mart bikes. The Target store near me has some decent-looking Schwinn hardtails suitable for bike-path and neighborhood riding that I believe to be decent values for the dollar. But that fully at Walmart? Just don't go there.