The most similar tank size to a 20oz CO2 tank is a 1.1L air tank. With gas hungry markers, Shockers especially I would recommend a 1.5L bottle in order to get a decent amount of shots out of it.
Air is a much more stable source, and does not fluctuate much over the chronograph when firing. Air is not as volitile to temperature either, and it is also more cleaner than CO2. All in all, air/nitro is easier to use, and is the preferred choice because of its greater consistancy. The only major drawback is the cost of air systems compared to CO2 ones.
Most new starters stick with CO2 because of the low cost of setting up. Hope this helps.