I haven't used the air gauge, but I've seen it used (example the youtube video linked above) and I have a pellet gauge that I use. I talked to one of the guys that invented the air gauge and know its very accurate, I won't speak for him, but its higher than we need. Of the two, they are likely about the same as far as speed goes. The air gauge is going to be way more accurate.
One of the things I've found with the pellet gauge is that you will often find pellets that are not exactly in a step size. So for example lets say I have a these steps... 7.65mm, 7.64mm, 7.63mm and 7.62mm. The pellet drops easily into the 7.65mm hole, the pellet drops into the 7.64mm hole with a very light tap on the skirt (mostly to align it), and the pellet will drop through the 7.63mm hole but with a strong tap, and requires exact alignment to push back through the hole, and will not completely pass through the 7.62mm hole, but it goes most of the way on the head...
The pellet gauge is a "go, no go" tool. Ideally the pellet should just fall through the hole without having to "tap" it. So using the ideal, in the above example the pellet is a 7.64mm, but... the reality is its really somewhere between 7.64mm and 7.63mm, but now that you've forced it through the 7.63mm hole you likely "trimmed" a high spot off the head, so now its a more or less 7.63mm head size. With the air gauge you not going to get the "hmmm" factor. You're not going to jiggle it around and get a different PSI reading. Using the pellet gauge is more of a art than a science. Used carefully with no bias in your use its pretty darn close. But if you want mindless fast and accurate get the air gauge.
Personally I'm leaning towards just resizing pellets to a size that works best for my guns. IDK if it will work or not yet, will be putting a bunch of resized pellets to the test tomorrow. One of the big motivations for me was sorting (10) 150 count tins of JSB 50gr 30cal pellets only to have ~85% of them too large for my gun. Literally I ended up with a hand full of pellets out of (1500) pellets that would work well in my gun. Total waste of money! So I invested a couple hundred in a press, dies and a table. If its a failure on resizing, I can still use the press and table with a air gauge lol.