Definitely recommend a 25 watt. I have a Cascade 25 watt and have no complaints. Be warned that any adjustable heater will NOT have a reliable temperature scale on it, so you will need to regulate the temperature until you get a feel for the way the heater works. For example, my tank is at 82, but the heater is set at 86.
I keep my tank at around 82, but that's usually considered to be on the higher side. Rosie just seemed grumpy until I notched it up.