It will all boil down to whether you need/want the extra features which the M3 brings, which you will be paying a premium for.
They are both excellent guns - accurate, good power, good shot count, smooth actions. M3 brings you a larger magazine and more external tunability. Do you need that extra tunability or are you the type who gets your gun shooting well with a chosen projectile and then leaves it alone?
FX says that even though the M3 has a smaller plenum, it can make more power. Bigger internal valving and air channels, I guess. AND it makes its highest power when you add the slug power kit - if and when that is available in your region.
Mav is simpler overall - less to tune wrong and less to wear out and have to replace (number of o-ring seals, etc.) over time.
I have a Mav .30 Sniper, and that gun has surprised the heck out of me (in a positive way) out to the 100 yards I have shot it. Accuracy and easy shootability.
I also have a .30 M3 'standard' (600 mm barrel). I'm definitely not where I am with the Mav yet. So much tuning to do with an Impact. It also didn't help that the gun was delivered with a .22 cal magazine in the box... Still waiting on the replacement.
I think that the jury is still out whether FX has gotten their manufacturing act together on the M3 platform. I am still seeing too many posts about the first regulator showing full bottle pressure when the gun is filled. And that is after the fiasco of drilling holes too deeply into the block and causing leaks which I guess were initially rectified using thread sealer. The very first Mav I purchased had a tendency to suddenly dump all of its air during a shooting session and went back to the dealer for a refund. Fortunately the new .30 cal gun doesn't do this.
If FX gun availability is so poor in Australia, how is FX service?