I don't think one is better than the other. It really depends on the materials you have at your disposal. Most people go with the cylinder since many of us is retired propane tanks.
correct me if im wrong, and this is just my 2 cents, but teh way i see it, with a cylinder shape, when a smoker is warming up, and the metal "sweats" it will roll to the sides and down, with a square one, it would prolly do this too, but would drip, just my way of thinking, prolly over thinking it, but thats how i see it
I asked the same question, and down to the fact I couldn't find any lpg tanks, I decided to build a square one, but with a cut off corner to make it look nicer.
That's pretty nice looking. I was thinking octagonal for my next. On the other hand, I have a place close to me that can roll steel so that's an option. But I like that with the corner cut off.
You can use the pit calculator on the main page of this site. I would start by entering the dimensions of the round one that you want to build. Once you come up with the cubic inches of the cook chamber then you can enter some dimensions for a rectangle and keep playing with them until you get close to the same numbers.