You seem to know what to look for. In a cheap bag, you will probably not get the long lasting quality that a top or middle tier bag would give, but the technology is so advanced today that most any bag from a legitimate outdoor company should be very usable.
I have a Kelty down bag that I have been happy with. I would place this in the middle tier. It has held up well. That said, I have also had a Slumberjack bag that was also a middle tier that self-destructed on it's maiden voyage. It was exchanged for a different bag altogether (Jansport). Many people have been perfectly happy with their Slumberjacks - not meant to discourage purchase of their products if you find a good deal.
In other words, cheap can be fine and serviceable, but any bag may be good or bad. You increase your chances of good with more money is all.
My personal choice of the two you cited, would be the Kelty, based on my complete satisfaction with their products in the past.