The choice is yours. Both are great stats (I have two Johnsons that work great), and the only real thing between them is that the Johnson is cheaper, but it's 'on/off' style (meaning it will turn on to heat the pad to the set temp, then turn off when it reaches that temp) and it clicks.

If you want to save the $20-30 for something else for snake keeping, then get the Johnson. If you want the best stat you can get right from the off and can spare the money, then get the Herpstat.