I used the 28 watt -- but that was because that was the only one that would fit in my cage. Your cage is larger, and you have more space to heat so I would probably go with the 40 watt. But you can always call Rich at Reptile Basics and he should be able to help you out as well.
And if you do get a heat panel this will also need a thermostat or a dimmer switch at the least.
Good luck!!!