Depends on -
The leverage ratio of the frame
Your riding style
You're willingness to play around with the settings until they're just right
The desired total weight of the bike
Coil shocks still have the edge in overall dampener performance when it comes to being both small bump compliant and able to take big hits. Air shocks are more easily adjusted for changing conditions.
There are pages and pages that you can write about this subject.
In the end it boils down to your particular needs.
I have a coil shock (CCDB) on one of my fullys for the best big FR/DH perf possible. My shorter travel bike runs an DHX-Air so that I can 1. adjust it firm for DJ's or soft for DH. 2. keep weight low. It helps that the frame has a 2:1 leverage ratio so I'm able to get very good performance from a DHX-A.
The whole equation is very specific to you and your intended use.
I've had extensive time riding both dhx coil shocks and dhx air shocks, the coil pretty much blows the air shock away when it comes to trail/dh riding, the only thing is the air shock pedals a little better, coil shocks are worth the extra weight.
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