I'd suggest using at least 2 heaters in case one fails. I'd also suggest using some sort of temperature controller and not relying on the built-in thermostats. Even the Eheim ones aren't that great tho the heaters themselves seem very good (they continue to heat for ages and without exploding etc).
If you don't have a full-blown controller this will work: (I love mine)
http://www.canreef.com/vbulletin/showthread.php?t=81882