cbr46,
"Stuck Bolts" have been an enemy of man, since the beginning of time. Many "tough" gents have been defeated by frozen bolts/nuts. The ability of a loosening agent, like PB Blaster, Kroil, WD-40, and a whole host of more of them, may or may not work. It all depends on just how much rust/corrosion has taken place and, for how long.
In actuality, I've not had a ton of luck with loosening agents. But, that's just me. Anyway, air tools, especially the lower cost ones, are generally over rated. The value of a good air tool, especially a big name unit like C/P or Blue Point (Snap On) or MAC, etc. is directly related to cost. You get what you pay for. The techs at dealerships etc. need to get their work done and not goof around with chincy tools. So, they PAY.
Now, a DIY kind of guy like you and me, well, paying upwards of a $1000.00 for 1/2" impact gun is not in the picture, never will be. So we get the best we can afford. The tools you're looking at as potentials, are listed at moderately stronger than the one you already have.
If it were me, and you think you might be encountering more bolts/nuts of that "stubbornness", I'd go for nothing less than about 600-700 ftlbs of torque on gun. That's minimum. Yes, you might have to pay a bit more than you'd like but, you'd have a tool that will work for all of your future projects. Just a thought here.
Scott