What Eric Alan said is spot on.
The last point there, and I could be wrong, is the most important.
When a Herpstat fails, something about the way it's wired means it always fails in the OFF position. Other thermostats fail in the ON position. This pretty much eliminates the need for a failsafe secondary tstat. Perhaps the other manufacturers are doing the same now, but this was the case a while back and was pivotal in my own decision after reading threads about the horror of thermostats failing in the ON position.
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