I would approach the issue from a matter of function. Buying on the cheap (regardless of the material) does not take into account what you want the pot to do. It's more than just volume and material. Does it have a lid? Do you want to install accessories (ball valve, sight glass, kettle screen)? Do you ever want to do 10 gallon batches? Does it have handles? Do you like a flat bottom like on a restaurant pot, or a domed bottom like on a converted keg? Is it durable? Does it fit on your heat source?
You should ask yourself these questions before you buy because you might find out later that these questions are more important that saving $20 on a cheap pot. Nothing worse than buying stuff twice. As far as materials go, either one is fine as long as the pot is useful to you and it does what you want it to.
Lastly, on an extremely biased note, you can't find a better boil pot than a coverted keg. The domed bottom allows extraction of more wort, they have great built in handles, they are easy to accessorize, 60+ quart capacity, and they are cost effective considering all the benefits. Think hard before you buy and materials be damned.
Prosit!