A rotor from the rear of a an all wheel disk car has about a 2-3" depression that that acts as a brake drum for the emergency brake setup -- This is what I use as a firepot and it works great, and it sits nicely 'in' my forge table without any mechanical fastening or welding -- just the lip of the rotor.
Even a 'normal' front rotor will have a 1/2 -2'' depression that goes over the wheel hub assembly. Don't want to create an argument, but the OP posted a legitimate question.....
And my answer is -- at least my preference, go with a rear rotor from a light truck/ sedan, if you plan on setting it in a table like a firepot. BUT -- -
If you plan on building one of the brake drum forges without a table (just a drum on a tripod of sorts) then go with the bigger drum firepot.
Whatever you choose, be sure to make the pot more bowl shaped with some woodstove cement or equivalent.....It made fire managment and heating steel much easier for this novice blacksmith. See my brake ROTOR forge attached.