Your booster pump along with the proportioner will allow you to get a stronger mix on a surface while at the same time giving you the ability to zero in on a precise mix that you need. So if you need a 10%, 20%, 30% mix etc, you have the ability to do that without guessing. Down streaming your pretty much limited the flow of your pump (GPM), length of hose, injectors placement, etc etc. When using the proportioner you will need to either have an extra hose for rinsing, or switch your booster to plain water.....for down streaming though, you can either change nozzles or use a remote control for rinsing, and not have to go back to the truck.
We do both, we have a proportioner (and we love it) on our dedicated pump system, and we also down stream a lot to. It just all depends on the job and surface we cleaning. For some like Ray Burke, I think all he uses is a Booster Pump and proportioner.