The general rule for heater size is 5 watts per gallon, but lots of things to consider before you get a new one. Most heaters are designed to raise tank temperature only a few degrees above room temp. Keeping tank in a cooler area, you would want to bump up in heater wattage above the general rule to compensate. I have several Jäger heaters, and a few marineland visitherms and have not had any problems with them. Depending on your tank setup, if your using a canister filter consider using an inline heater, they are supposed to be more efficient and less clutter in the tank. Get the best you can afford, it's worth the investment.
I was thinking about getting an inline heater but, I already have a reactor attached to it and I feel I shouldnt' attach anything else for flow rate problems. I just have a xp1 running, in hinsight I should of got the rena filstar xp2.
I think one of the best I use is the fluval e series. It has two computer chips on it and digital read out. Also low flow sensor and shut off it you leave it on and take it out of the water. Another thing the background has three colors on it.
Green = the right temp
Blue = water is below temp
Red = water is too hot
Flashing Red = Problem with heater
These heater stay very steady. I own 3 of them.
I'll have to look into this heater.