i have owned both i like the cm9 it is smaller and much lighter it makes conceal carry much easier
My PM9 is, without a doubt, the finest 9mm carry pistol I have ever owned. As a bit of history, I have owned about 20 9mms since I started looking seriously at carry pistols, starting in 1983 or so.
I really love the HK P7 series.
I have owned a bunch of P7s, including six or seven P7 PSPs, two M10s, and one M13. However, they are very expensive (especially the M13, which is, IMHO the finest P7 ever made) and all of them can be somewhat ammo-sensitive.
Due to the design of the gas/piston recoil system, anything other than a fully jacketed bullet can force molten lead through the gas piston port and cause a slowing or stoppage of the action. Yes, I have had it happen.
Best info I have found is that the CM9 and the PM9 are identical in frame and slide...sort of. The rifling in the barrel is different...conventional (CM9) vs. polygonal (PM9). And the CM9 lacks the slide sight dovetails that the PM9 has...so replacement sights may be an issue for CM0 owners.
The difference in weight of the two should be a non-issue.
My PM9 is approximately 14 oz, and the CM9 should be nearly identical.
Bottom line: I would buy whichever my budget allowed, and would not stress out over my choce.