Thanks again for the input.
I think I will go ahead and get the bigger tub.. As you said, he can stay in it longer without being cramped and he'll more-than-likely use the space. At least he'll have the option to climb (I intend to put some wood in the setup for climbing).

Tlich- Is the lamp you use on the "cool" end of the tub and is it over a wire mesh insert, or just the normal tub top? I think using a lamp on a dimmer would work for me, but I"ll have to find one with a stand.

Satomi- Do you think using a RHP would be more advisable than a lamp? I can't raise the ambient room temp because I'm on central heat / air which would mean heating my whole apartment (and I can't stand being hot)... I'll have to go with a localized heat source of some kind.

Thanks again guys for your advice! I plan on buying the tub this weekend and have already ordered the UTH and hides.