Herms HEX Temp Contol: Bypass or Burner?

Am currently working on my design for a herms tripe brew rig. I will be using a herms loop through the hlt. My question is, is it better to control the mash temp by using a pid to control a bypass valve to divert the wort through/around the heat exchanger; or is it better to use the pid to control the burner on the hlt to maintain the hlt at the mash temp? Also, I know with the burner control the temp probe should be at the hex outlet, where should it be with the bypass type system?

Use both, a tank of hot water is difficult to maintain a precise temperature and it's the temperature in the mash tun that's most important. Electric RIMS are far easier to build and control (just a thought).