I don’t really know how good the quality is for those Walmart bikes, though I can still give you some insight based on my engineering background.
You should always go for a lighter weighing bike. The reason is energy. When you ride a heavy bike, you will have to use more energy (from your body) to drive that bike and therefore get tired quicker.
So basically, you provide the fuel that drives the bike. And the heavier the bike is, the more fuel it will consume.
Generally, aluminum is 35–45% lighter than steel. And the frame constitutes majority of the bikes weight. So it is the most part of the bike that you want to be lighter.
Hope this helps. Let me know if you have any other questions :)