I would go with a 100 watt. You might need to keep it at a lower setting but it will work just fine and will not work as hard as a smaller one. I have a 400Watt in my 80 gal It came with the tank. Its over kill I keep it at the lowest temp and it does not come on very often and thats good because it will use less power. I run a 100 watt in my 20 gal tank and it does a good job I any lower and I found it hard to get the temp up to 80F.
I find a heater with a bit more power is better.