Have you looked into Sera heaters at all? I've not owned one, but have heard they're built very well. I have a mix of marineland, aqueon and various no-name cheapies. I tend to use lower wattage units in the event they stick on as mentioned above. At least in the US, most of your store bought heaters, including marineland and aqueon have lifetime warranties, but you've got to be sure to hang on the receipt and maybe UPC to get mailed a replacement. I've gotten them to send me new heaters with just an email or two, w/ a pic of the dead one along with the receipt. I've got enough spares to last me a long time.
My tanks run between ~70F into the low 80s in the summer time (no a/c where most of them are), so basically the heaters don't run at all during the warm months and only have to heat the water a few deg F warmer than room temp in the winter.
I know heaters are designed to be submerged, but I get longer life out of them by leaving the dial-ends out of the water.