Herpstat.

My first rack uses a ranco, which has not failed me. Although the tub temps are between 90-93 on the hot side, I have to set the ranco at 100 to get those temps, and the heat cable fluctuates from 98 to 103 to get the 90-93 temps in the tub. I recently got a herpstat for another rack and it works like a dream, I set it to 95 and the tub stays an even 92.

Id use a herpstat, esp if its heated using flexwatt, (less fluctuation, less chance of screwing up the flex-watt)