About all big tanks are good for is using air tools. They don't do much when airing up tires. The volume of the tank gets dumped quickly into the first tire and then you're just on the pump. A small tank for a little buffer for the compressor is a good idea, but not much is needed. The difference in overall air up times with or without a tank is very small.
As for running air tools, I don't think it's worth carrying the weight. There isn't a trail repair that you can save a significant amount of time on using air tools to justify carrying them.......in my opinion. I used to carry them, but I never really used them, and less weight means more performance and less breakage. By the time you get your air hose and impact out, I'll have grabbed my star wrench and have the wheel lugs off already.
I've had both a QuickAir2 and now an ExtemeAir, and I am happy with both. The ExtremeAir is a heavy duty, 100% duty cycle compressor that works very well and should last indefinitely. Some like the CO2 tanks, but to me it's too much hassle. I could care less about portability and air tools, and I don't want to bother having to fill it occasionally......keep it simple, there are enough other things that have to be done.