The inside of either of the smokers is aluminium, the exteriors of both are steel. The black one is a baked-on powder coating, the stainless is, well, stainless.
The only difference in the two is the asthetic value, which isn't worth the extra money in my opinion. If you abuse stainless, it will rust. If you abuse the black coating it will rust. I smoke in rain & snow, take moderate care of my smoker & it holds up just fine.
Also, I've heard the stainless looks great for a while, but becomes high maintenance to keep looking good after a while. My black one still looks great after two years of hard use.