Submersible heaters are more expensive, but much more convenient - you don't have to worry about water, coming inside. 2 x 50W heaters for reliability.
I have these only for less than year (after Visi-Therms, that are still working in other tanks), like them:
- All-Glass Submersible Heater 50 W (Hydor made) - the smallest,
- Visi-Therm Stealth - (in a big tank) they are not a glass, plastic onto aluminum - for mechanical durability, both have auto shut-off.
At first bought Eheim Jager heaters, as I was told they are the best, but mine has a line "Do not immerse below this line", and not a word about auto shut-off. Prefer the two above.