Commercial grade equipment is manufactured for the needs of industry. It's designed to be dependable enough for people to rely on it at their jobs, where folks make their money using the equipment. You'll find heavier steel cases, higher duty cycles, higher capacities and more rugged controls and equipment. You'll find this stuff at welding supply shops. These places are great, and often have a demo room where you can try the various models of equipment, at least our local Liquid Air dealer does.

Non-commercial grade equipment, which you'll find at home improvement and department stores, are designed for hobbyist or recreational/home use. Lighter cabinets, low duty cycles, lower capacities, and lower durability.

That said, even the home owner grade equipment sold by the larger manufacturers is quite useable. My Lincoln is such a beast, and it performs very well. A local farm supply store sells Deca welders, made in Italy, also mostly light duty stuff, that also works quite well.

I have used a Liquid Air branded Lincoln (I think it was a lincoln) of similar specs to the one I now own, but commercial grade. It was clearly more rugged, and would outlast mine in "unloving" hands, and had a higher duty cycle, but in my hands the results were indistinguishable.

I'm going to give advise contrary to what quepasa has given (variety is the spice!), and suggest a non-commercial welder if you're interested in a Mig unit. Few hobbiests need high duty cycles, and unless you're the type to abuse your equipment, it'll last plenty well. You can put the 200 bucks you save into metal and supplies with which to practice the art.

Have fun!

J