I might suggest 2 heaters versus one. Here's my reasoning. When heaters fail one of two things happens: 1) It fails on...boiled fish. or 2) It fails off...cold fish. Personally, I prefer the 2nd outcome. However, all to frequently...I've seen the first happen. Now, you toss in 2 smaller heaters and you do end up with less heating power overall, should one fail. But, should one fail on....it won't contain the power necessary to fry everything in the tank.

All the while, the other heater shuts down or works less than the one going crazy.

It's a win-win. And...you can still tell what's happening, as the spot around the bad heater will get really warm, and the fish will stay away from it.
I'm doing this with my 55 soon. Yeah, I set it up with one heater, but have been seeing the lovely ideas of two. Every site I see that has one states much less problems with heating the whole tank, as you have more heating the tank than one heater can provide by itself, and they say that the overall temperature of the tank is much more constant than with just one heater.
Just my .02 cents.
