IMO the 6MM should have won out, just like the .280 Remington should beat out the .270 Winchester...but due to being beat to market neither one of them did. Now it is harder to find guns chambered in those cartridges than it is factory ammo for them. 6mm is almost imposible to find a gun chambered for it, unless you go the custom rout or rechamber an existing gun.
If you kind of like the idea of an obsolete cartridge that is a little better than its competitors then the 6mm is the way to go. The .225 Win has been obsolete for years, but they still make brass for it. Or you could step up to the non-obsolete, but not as wildly popular as the .243, 6.5x55. Not much affordable varmint factory rounds for it, but again you can reload.
If you kind of like the idea of an obsolete cartridge that is a little better than its competitors then the 6mm is the way to go. The .225 Win has been obsolete for years, but they still make brass for it. Or you could step up to the non-obsolete, but not as wildly popular as the .243, 6.5x55. Not much affordable varmint factory rounds for it, but again you can reload.