I have a twin ARB and 35's. I usually air down to 18 or 20 psi and run my street pressure at 38 psi. I can't advise on the single but the twin is very fast. I'm aired up from start to finish in about ten minutes. The speed also encourages me to air down when I'm not sure how rough the trail will be.
This.
I run a Puma 12v, which is roughly 85% as fast at nominal tire pressures as the ARB Twin (~3.5 CFM @ 40 PSI for the PUMA vs ~4.0 CFM at 40 PSI for the ARB twin). It takes me about 5 minutes to inflate my <31s" from 15 to 35 PSI, plus about 3 minutes of setup and teardown.
I have experienced and observed using "perfectly adequate" $50 air pumps. It is easy to say "for the once a month I'll use it, I don't mind spending an extra 10 or 15 minutes inflating." However, I have found I am much more likely to just go ahead and air down even when I am just on dirt or forest service roads, because 1) it really smooths out the ride (for both passengers and the vehicle), and 2) I know it won't take much time to reinflate (7-8 total minutes is an easier sell than 18-20+).
Of note, I ended up getting the Puma because it is about 40% of the cost of the ARB twin despite, again, ~85% of the total airflow. That is, it is both notably cheaper and significantly more powerful than the ARB Single. Plus, it's tank makes it more capable of running air tools if that is valuable to you. Still, in light of what I said above, and given I use all cordless power tools now, I'd have probably gone with the ARB twin for its relative compactness. If you have the space, though, it is really hard to beat the Puma 12v for the value.
