The general rule that has always been quoted to me about heaters, is 3 to 5 watts per gallon. If that sounds familiar, that is about the same thing people say about lighting too.

You have 100 watts for 29 gallons, a little more than 3 watts per gallon. If you go up to 150 watts, you will have just over 5 watts per gallon. And in reality, it does not matter if you go over, all it means is that the heater will heat the water quicker. I suppose that the chance of a fish getting burnt is higher of a higher wattage heater, but other than that, I can't see any problem with getting a bigger heater than you need.