Most coil-over shocks have 4 or 5 presets for preload which gives you some flexibility. Air shocks have unlimited preload adjustment which allows you to find tune the ride to the exact load.
That said, when I had to temporarily use a set of coil-over shock while waiting on air shock replacements to arrive, I did not notice any appreciable difference in the ride. Air shocks, particularly on a trike where the shocks are not out in the open, would make it easier to adjust but I doubt would improve the ride quality noticeably.
As a side note, keep in mind that adding air shock introduces more potential for problems such as air leaks, shock seal blowout of the air chamber, etc. Coil-over shocks are pretty much bullet proof.