Brew Kettles

I have been looking at brewing kettles, and I could use some advice. I am looking for something around 15 gallons, so that I can do all grain batches of at least 10 gallons. Northern Brewer has a Polarware 15 Gallon Kettle made of 18-8 stainless that would work. Midwest has a 14 gallon stainless kettle with .8mm sides and 1.0mm bottom which is quite a bit cheaper, but I would question the durability of this compared with the heavier kettle. I know that a lot of people have converted used kegs for use as brew kettles, but I have not found a good source for that option, and I am not sure how much modifying I want to try to do. Any advice would be appreciated. Dave