To me the Fox 40 seems much better at damping control and sensitivity right through the travel. It’s similar to Zeb vs Fox 38 in my experience, the Zeb felt move over damped in many situations and the Boxxer felt similar.
The Charger 3 in Zeb was decent but I could never get it to match the 38 for overall sensitivity and support.
Spring wise, the Boxxer requires much higher air pressure (160-170psi in my case at 80KG) versus only 80 PSI in the Fox 40 - so the Fox runs substantially lower pressures which must be due to the larger diameter air piston. I believe this might contribute to it feeling more sensitive also - bigger chamber, less pressure.
I think that this combined with the Grip 2 damper in the fork combine to make a more plush feeling ride throughout all of the travel - it feels like a beefy Fox 38, with the benefits of Dual Crown and the larger damper and air spring diameter.
I might add that I know others that prefer the Zeb over the Fox 38 - so maybe it’s just what I’m used to. But, IMO. Fox has the edge in sensitivity and damping throughout the entire stroke. Boxxer seemed to ride a bit higher, plush off the top, but felt more ‘choppy’ over constant chunk.
Saying all of that, I’d definitely be more than happy with either fork and it’s only as I’m in a position to try both that I could test back to back, but it took half a run to tell that I preferred the 40.