Humidity isn't as big of a deal as alot of people let on. If your snake is having bad sheds where not all of the skin comes off, then you look at humidity. Alternatively, if there is mold forming you can also consider that it may be too high.

Tub or Tank doesn't matter much. One thing to consider is the ambient temp. I would not recommend trying to have a heat lamp on a tub. If the room is at least 72 or 73 I would say you are fine with a tub and a heat pad. If the room gets below that, I think an xpvc or a tank would be a better option for you.