I have had both. Personally I prefer the Genesis. They were both supposedly the same size but I felt like I was a bit scrunched on the Mongoose. I think the different lay out just didn't really work for me. As to the Walmart assembly is crap type of stuff I keep seeing in your answers, find an employee and ask who assembles the bikes. At the store wear I live, they are assembled by a bike mechanic who also does repairs on bikes (I bought my Genesis used there).I don't know if that is store policy or if I just got lucky that they had an employee who had the skills for it. The Mongoose I'd bought at Sears/K-mart and they didn't have a mechanic who assembles it. The assembly is just done by who-knows. It was missing a rear reflector, and the chain kept falling off until the chain guard busted. I returned it the next day and got my money back. Walmart actually ended up being a much better place to buy a bike. Yes, bike shops are going to be the best, but financially you'll pay a lot more (cheapest bike they had at the few places I checked out was $299, and it was used!) Hope this helps.
PS: Walmart does not allow to test ride the bikes before buying, so if you want to do that, I'd suggest going at night when nobody's there to see you and order you off the bike. Speaking from experience here.