There are so many variables that can affect the choice of coils or air. I've always been a long travel, big hit MTB guy...SC Bullit, Nomad, etc...and I come from a long history of dirt motors. Because of the dirt motors, I've always been much more partial to coil suspension.
The main area on MTB's and EMTB's that I notice benefit from coils is trail chatter and small bump compliance. What?...why should that be a big priority on a long travel bike? For me it's always been the annoyance and even fatigue generated by the constant feedback of small trail roughness, chatter, etc., over the duration of a long ride. Air suspension doesn't usually do near as good a job as a coil setup in removing or at least diminishing it.
Now, I have had the rare experience of an air fork...Marzocchi 66SL...and rear shock...Manitou ISX-6...that came nearly as close to coil performance as anything else I tried...and I tried a lot. Working part time at a bike shop I ended up buying and trying all kinds of options. Still, overall I found coil suspension to be superior for me, my riding style, and preference.
Maybe another advantage considered by many, though secondary to me, is the benefit that once you settle on the preferred spring rate/weight on your bike, there is little to no putzing with setup other than tire pressure each time you go for a ride. Speaking of spring rate/weight, personally I think there is little advantage to spending big money on the lighter rear shock coil springs on an emtb. This was much more valid on a pedal-only bike where weight reduction was a huge benefit. Not so much for an emtb. Example, the Fox DHX2 rear shock I installed on my new Rail 7 has the standard steel spring that costs about 1/3 that of the lighter versions. I bought another steel Fox spring that is one step heavier that I'm preparing to test this weekend. On a powerful emtb I just don't think the more expensive, lighter springs are necessary. Progressive rate/weight springs can be a reasonable alternative if you find your bike's suspension would benefit from one, but that's a different situation.
