Portable Air Compressors

What tool are you wanting to run? That would be what should influence your decision. If it is something like air sanders or grinders that require a constant supply, I'd go with a bigger compressor. If it is just nailers and such that won't have long run times, expansion may be okay.

I've always taken the approach of buying "more" compressor than I think I need. It has saved me many times when I have a use for it. I have big compressor in the shop, and I have an older Kobalt compressor on the dollies that is easier to move. Also, I keep a small pancake compressor that I usually just buy used when they crap out.

ETA: I bought the Kobalt after working with a guy who had it. He ran everything off it it, and it was portable enough not the be a pain in the arse. He ran four pneumatic jacks at once off it, and it was quieter than the Dewalt pancake I had at the time.

Something along that line is what I will go with if I get a bigger one. I had a 5 gallon compressor before and between the frequency of use and the trouble it was to drag around I ended up replacing it with a smaller compressor and I am not unhappy with that decision at all.

The expansion tank idea would not be a constant use thing. It's for sporadic use when higher demand is required.