I have a UDS and a chargriller. While I haven't measured them, I think they are roughly same thickness (though I could be wrong). The problem is, it takes quite a bit of reinforcement when you split a drum down the middle to make it sturdy and leak proof. Personally I'd go with the Chargriller, or use the drum but keep it upright and uncut.
I don;t know what gauge you ordinary drum is but using alot of metal studs in my day I would say that they are about 20ga. or so maybe alittle less. Like Geek said you will really need some good bracing to make it hold up. But for what you will spend and then install bracing I would just look into buying maybe an old propane tank cause they are very heavy and would be a work perfectly for a smoker.
I have a CGSP, but was thinking of building a smoker from scratch. If that's the case I won't bother. I think I'm going to go to the local scrap yard and see if I can find a nice large propane tank to use. thanks guys!