I like the trunk mount to get the flammable meth (in concentrations over 50/50 with water) out of the heat of the engine compartment, to not render the washer fluid less effective since I drive year round, and to not tap into the windshield washer nozzle hose. For a max of 50/50, I don't think any of the above matters, but I want to run 75/25. I also don't want to zip tie my PS reservoir to my AC hose or have a relay hanging from something in the middle of the engine compartment, but those are lesser concerns.
With the trunk mount, you have a long line under the car, but I put the meth hose inside fuel or heater hose to help protect it. It does take up some trunk space, but it can be located on a side of the trunk. One or more wires have to be run under the carpet to the trunk and through the firewall to the Procede.
If you read up on meth systems on 135/335, you will learn that there have been a few engine fires from the meth. I don't have all the details or know what to blame, but I am more comfortable with the trunk mount.