I used the Phazer by EcoLab system on a new construction job months back and decided we had to go with the backpack systems. I looked at Betco, Rubbermaid and 3M before deciding the EcoLab was the best designed one. I was using someone else's the first time, which they had 3 of and 1 worked, so I figured out real quick how to take it apart and clean it properly.
The advantages I found with the Phazer was: 1) trigger design allows it to rotate 360 degrees with no hang ups; 2) Clean-out feature, comes with a 3/4" attachment (attached to bag fitting, not a separate hose like others) that will hook up to any janitor sink or 3/4" hose; 3) ground level dispensing valve, easy to take apart and clean, nothing fancy and you can clean it out in under 5 min; 4) Uses same type bag fitting as 3M and others, so you can get your own refillable bags and use whatever finish you want, not just EcoLab's.
Oh, did I mention we have gotten 3 for free?? EcoLab will give you one (or 3) when you order a certain amount of finish. Pretty nice when you look at the Betco or Rubbermaid and they want $500 for them! These two also have weird ways to clean out the system once they are done. Like filling up an empty bag of wax with water, then running it through the wand assembly. Really? I don't have time for that, when it can be as simple as a hose attachment.
The disadvantage (if you want to call it that) to the Phazer is the huge "turtle shell" on your back. I bang into crap constantly with it. All in all, backpack is the way to go, just researching them and talking to salesman who've never used them is always fun!