-Go with the wider, shallow tub. I have a 15qt tub that measures 16"L X 13"W X 6 & 5/8"T.
-The Under Tank Heater for a tub that size will need to be small. Either an UltraTherm or ZooMed in the 6" X 11" size, or an Exo-Terra Heatwave Desert in the 8" X 8" size. Or, ordering a piece of Flexwatt heat tape is also an option.
- Here are your choices for a proportional thermostat:
Reptile Basics- VE-200...$114.99
VE-300...$139.99
http://www.reptilebasics.com/thermostats
Helix- DBS 1000... $139.00
http://www.helixcontrols.com/DBS1000.htm
Spyder Robotics- Herpstat...$109.00
Herpstat II...$199.00
Herpstat Pro...$299.00
http://www.spyderrobotics.com/home/products.html