Feature wise they appear to be identical.
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Check out the product comparison here:
Here are the only differences I found:
Battery life: The Aimpoint PRO battery life is 30K hours where the Comp M3 is rated at 50K hours. (My M2 is rated at 10K hours).
Both the PRO and M3 have 6 Day settings and 4 NV settings.
I have a M2 and it's rated at submersible to 25M/80 feet while the M3 and PRO are rated to 45M/150 feet.
The PRO is only available with a 2MOA dot while the M3 you have a choice of 2 or 4 MOA. The M2 that I have is 4MOA.
On paper I can't think of a single thing wrong with the PRO.