Owning both, the 6.5mm Creed is the better choice for a casual long range shooter, banging steel and punching paper. The 6mm Creed is really the tool for the PRS gamer, where we have an expectation of high round counts and shoot from awkward positions (meaning low recoil is important), and where we're capped with a 3200fps speed limit.
The 6mm Creed can shoot as flat as the 6.5mm, or flatter in many cases, as the 6.5 creed just doesn't have the case capacity to put the speed behind the heavier bullets. The 105 Hybrid doesn't give up much BC to the 140's, and the Creedmoor case can get it up to 3150fps, as opposed to fighting to get up over 2850 in the 6.5mm. The 6.5 carries ~20% more recoil to get the job done, which is a big driver for why we're seeing 6 XC and especially Dasher gaining some popularity in the sport.
But for match work, the 6mm Creed runs right up against the speed limit, without a magnum bolt face (mag's prohibited at some matches/ranges), and without excessive recoil. The downside? Well, I'm going to burn up my 6 Creed barrel before season is over, whereas I'm still going strong on a 2yr old 6.5 Creed barrel.
If a guy is happy to replace barrels, then the 6mm Creed is a good match option. For the average Joe who doesn't know any better, but really wants to look cool with his RPR, the 6mm Creed isn't a great option - although it'll likely take him a handful of years to get over 2,000rnds anyway, and he might not actually be capable of shooting well enough to notice when the barrel burns out.
In fairness... It doesn't burn barrels as fast as 243win, 264rm, or 6.5 RSAUM (or 7wsm, 7RM, 300wby....), so maybe it's not all bad after all. I venture the new 6.5 PRC will scorch barrels in a flash too.
My long distance, non-hunting, pleasure plinkers are Dashers.