With a heater in a turtle tank, you have two considerations. First, you need a heater big enough for the tank. That will partially depend on the temp of the outside air.
Second, you want to make sure your turtle won't get hurt by it. I started with a cheap glass one. It worked fine. When I moved over to a larger tank, I went with one that our turtle can't break, even though I'd had no issues with her attacking the heater.
The one I got is rather expensive.
http://www.amazon.com/Fluval-E-300-Watt ... B001VMSK0I. Others have put PVC tubes with holes drilled over the glass ones.
I've not seen much that would imply there is a big difference between a cheaper heater and the expensive ones when it comes to heating a tank. You can get the pricier one like I did, which a turtle isn't going to break. I don't have a thermometer in the tank because that heater has a back-lit LED that shows the temp. You can get even pricier titanium ones that won't break, even if you pull them out of the water when they are plugged in.
If you don't want to spend a lot, you can get a much cheaper glass one and do something to protect it down the line if you notice your turtle bothering it.
Probably the biggest thing is to make sure you have a gfci plug for the stuff plugged into the aquarium.