1) I have found the Quick-Jet to be the biggest waste of money I every tried to save. Wish I would have just gone straight to the Tree IV. I agree...those parts do not justify the cost, but it is what it is. I has paid me back just fine...but I am getting paid to treat trees, not just doing my own. Still think the Quick-Jet is not (quick).
2) Not sure the Q Gun is any better than the Quick-Jet. It is designed better, stronger components, and between the two that is the one I'd choose if I had to.....but I wouldn't if I don't have to.
3) Rainbow's Q-Connect would be the other choice before either the Quick Jet or Q Gun for me. If I didn't already have the Tree IV, I'd weigh between the Q Connect and Tree IV:
http://www.treecarescience.com/treecare-products/q-connect/?cat=tools-equipment It is built better than the Tree IV, but they aren't giving those away either (costs more than Tree IV). I also like the fact it is easier to use the injectors in multiples less than 4...not a big deal with the Tree IV, but it is nice. Not using the plugs sounds good...at $0.50 a piece, they do add up! But nothing leaks out of the plugs either. I have only one time experience with the Q Connect as a demo, and a little leaked out of the holes....not a deal breaker, just something I noted. Less likely to blow off some bark with the Q-Connect (if an Arbor Plug is not set right, you can get product that seeps behind the plug, under the bark and damages a small area). If you like the Tree IV but no plugs, you can get it with Stinger needles instead of Viper needles. You might call to ask if they'll ship it without the viper needles including the stingers instead and maybe reduce cost??? (would still cost more than the standard kit, but maybe they won't make you but Viper needles???)
4) With some of the relatively smaller trees you have, soil drenched imidacloprid may not be a bad option. If you start EARLY, it works. Use the highest labeled rate (most labels now allow for a doubled rate on trees larger than 15"). Apply right at the base of the trunk (dig a drench around the root flare and pour it in there or inject it right against the trunk). TREE-age does work better, no doubt about that...but it costs a lot more - both to get started and to apply. It sounds like you have quite a few trees, so the thing you would need to watch is your maximum product per acre with a soil drench.
PS: are you a homeowner, or arborist? TREE-age is restricted use, meaning you need a pesticide applicator's license to legally purchase...