Some of the differances would be a nicer burner giving you a nicer flame, higher efficiency, more heat output...just a better build in general. With any appliance, you get what you pay for. I would say Monessen is a nicer unit then Comfort Glow, though there are better manufacturers than Monessen as well. I would do a little more research at other local dealers (carrying other brands) prior to making a purchse. You are also much better off going to a specialty store being they will most likely stand behind their product and offer service. Doubtful the Lowe's Staff will even have the knowledge to trouble shoot any issues over the phone, let alone sending a tech out to resolve it.

(Sorry, don't have any direct experience with either brand).

My $.02

Comfort Glow is a DESA product, not terrible products but not the best support either. Monessen has come a long ways since they began, and inserts are slowly fading away anyway from what I see. The biggest difference in price is buying power/markup. Lowes buys a lot for less, specialty buys a few for more. A monessen insert costs me as much a lowes unit off the shelf I bet. between the two I'd say for the most part their about even, but I'd have to see them(different models) to say for sure. Desa does a lot with manufactured modular homes and is not sold by any fireplace store that I know of anywhere. Most times I can't even get parts for one unless I call Desa directly.