I think it is largely based on the situation. The constant fire temple is good for a guaranteed source of damage and no reload time, but has poor damage. The crypt is good at dealing heavy damage in a burst at the begining of a wave, but has a slow reload time. The den is the most well rounded, dealing decent damage at a steady rate. I think in general the den wins, because of this balance.
Recent posts by fujiyamyam on Kongregate