I love the 6mm cartridges as well, my first centerfire rifle was an old Parker Hale that I bought at 18 years of age with my own money. Still love the .243 and have a few of them in the safe, but I've started to gravitate to the smaller 6mm cartridges. I've been shooting the 6X47 (6mm-222 Rem Mag) for almost 10 years now awesome performance out of a little case, only complaint is I'm limited on bullets by a slow twist. Pretty soon I should have a 6X45 once it gets back from the Ceracoate applicator, it has a 1:7 twist X-Caliber barrel on it and I picked up nearly 1000 Sierra PH 100 grain bullets for $40 awhile back that I'm going to try through this rifle also have 500 Speer 80 grain HC bullets if the Sierra's don't work.
The 6mm CM has my interest as well, but it's hard to beat a plain old .243 Win for general purpose deer, pronghorn, coyote, and occasional varmint rifle. The CM is a lot more specialized to long range shooting than the .243, though the old .243 is plenty capable of it in the right rifle. I don't know what the Marlin XS used for a twist rate, but the M700 and the Savage 10 both have a twist rate fast enough to stabilize the old 105 A-Max and BTHP from Hornady and you can send those a long ways. I was interested in the .243 Competition Match as well that Chad Dixon at Long Rifles Inc. is hot about, but since the 6mm CM is now commercial I'd probably just opt for it over the .243 CM.
