That small, I wouldn't go any less than than .100" especially if you will be wheeling it around. A RF smoker will probably weigh a little more than an offset because of the BP. I used 2" x .100" tube for my legs on my RF , CF smoker, and that was plenty strong enough. zero leg deflection when moving it around. But metal has more than doubled in price recently, so the heavier the metal, obviously the more spendy it becomes.. If yo decide to go with 1-1/2" and you get deflection, you could always add a few gussets after the fact to keep it from moving or twisting
Minimum thickness of 1.5 inch tubing is 0.065 and would be more than adequate for that size pit. Smoker legs are short and carry the weight vertically so a heavy wall is is not necessary.