The poorer quality bag (duck down) will have a much shorter life. Also, the Kelty bag has an EN13537 temperature rating, which means it has been laboratory tested so that the temperature rating is pretty accurate. I'm sure the Walmart bag does not have this feature, which means its temperature rating has been overrated by at least 10*F (normal for cheap sleeping bags, not just at Walmart).

Re sleeping bag length--I discovered during my bout of plantar fasciitis last year that when we relax in sleep, our feet go into the pointed toe position (why a night splint is needed for that ailment). That adds about 2 inches to your body length. If you sleep stretched out on your back, you should get a long bag if you are 5'10" or over, or you won't have enough foot room. Of course if you always sleep curled up, that won't matter.