I too am curious about the difference, but mainly because of the heating issue! In the hospital with my first baby, I spent a good 4 hours in the tub. They have an endless water supply, so keeping the tub warm was not an issue. I, however, in our little 1924 house, do not have an endless hot water supply. Dh and I can't even take back to back showers. I can't imagine the tub staying warm enough for 4+ hours, so I am leaning towards the aquadoula.
In the spirit of not stealing your thread - you can find the dimensions online for the aquadoula and a blow up pool. I remember looking at it myself, and I think there was only like an inch difference between the two.