I somehow missed this thread

I'll chime in a little bit since I have a lot of experience with both a foam gun, and foam cannon.
I've been using the Gilmour (Pinnacle) foam gun for about the last 8 months using a variety of foam/soap solutions. I had settled on using DP Xtreme foam after trying many many soaps. The gilmour produced great foam, however, after about a minute, the foam has come off the panel, or you need to keep foaming the panels as you go along. The best way I can describe the texture / density of the foam is "frothy".
Now, I've got a 2700 PSI Pressure washer along with the Foam Cannon attachment that SuperBee generously helped me source (hey thanks again!). I've been using this for the last 2 months. Using the same soaps (DP Xtreme) the foam produced are totally different. The foam produced from the Foam Cannon (via pressure washer) not only sticks to the panels FAR FAR longer, but the texture made from the foam is much "creamier" if I had to put a word to it. Think shaving cream, but slightly heavier in texture.
Which one is better imo? Well for me, I not only find the Foam Cannon much more fun to use (it's one of those oddly satisfying things like popping bubble wrap) but it also cleans MUCH better. The whole idea of having the foam cling to the panels, is it allows the soap to do it's job better. The longer the foam suspends against hte panels and can work against loosening up the grime etc, the cleaner your car will be with less scrubbing or chance of marring.
I had planned to do an extensive comparison between the two, but simply haven't had time to sit down and write it yet. Let me end with this, if you want SERIOUS foam (and fun) go with the foam cannon and pressure washer. If you're on a tighter budget, there is nothing WRONG with the foam gun, it still accomplishes the task at hand, but there are better (foam cannon) options available imo.