I'm also a fan of Ultratherm UTHs (although I use RBI UTHs in my RBI vivs). Unregulated an Ultratherm will still get up to 110 degrees.
As Deborah said, one of the features of Herpstats is that if they fail, they fail in the OFF position. Others fail in the ON position. Whatever you get, be sure that it isn't one of those. Better to have your snake a little chilly at room temperature than burned or killed by a runaway heat source.