The pad should not go on the bottom at all.. while I dont agree at all with bshmerlie's idea of putting a heat pad on the back wall its is the better option if your only going to consider the two..
The problem with tank mounted analog thermometer is your not getting the core temp inside of the tank.. just the temp of the glass where its mounted. Making it inaccurate.. a digital thermometer with a probe is much more effective and are fairly cheap anymore..

Heat pads and or tape is just a bad option for arboreal vivariums. and in my opinion you should look into a bulb swap before you decide to go that route..