A lot depends on the size of the enclosure you are trying to keep warm. In my setup, ive used 40, 100 and i think 150w CHEs at various times.
If you have a small aquarium, enclosure, with a good lid, 100 is probably good, but make sure you use with a thermostat. Check out the 1000w thermostat at LLReptiles.
Most of the time, the 100 is good enough. You will have to test it with your set up. Lots of times, more then one is needed for night heat when the basking bulb is off. Two 100 watts are better then one 150 watt, as one 150 will heat it's area, but not the other end before the end the che is on gets much hotter.