Part of the decision on which tub to use also stems from what substrate you decide to use. In my case I do the substrateless method so my tubs can be are small (6QT where the sides are only 5" or so).

If you are using vermiculite then it pays to have deeper substrate which holds moisture/water longer/better

Also the more mass you have the better the temp stabilization. (I believe Adam that plans to use 5 Gal buckets in the coming years so he has lots of substrate (ie. mass) and his temps and humidity stay rock steady)

Hope this helps