You might as well just go with a herpstat or other proportional t-stat. Only about $30 difference. If you want to save $20 you could go with the johnson and a rheostat for safety incase it fails and to not let the flexwatt ever have full power applied. I've read that helps, though not as much as a proportional t-stat.
The herpstat has, I believe, a failsafe incase of failure. Although I've heard if the temp probe come unplugged it goes to full power. So you may want a rheostat reguardless.