If I was limited to factory ammo, then the no brainer 6.5 CM.
I used one for a few years, until I shot out the barrel and needed to change it up. My 15 yr old daughter wanted to go shooting with me, so I let her try it out with the 6.5. She was comfortable shooting to 1000m easy, but the recoil was just a tad to much to handle and be comfortable, and this being with a 18+/- lbs rifle. Granted I was pushing 139 gr lapua bullets at 2850 fps from a 26" barrel.
I decided I would get a 223 for her. Lots of fun, and a bit cheaper to shoot and reload for. Great out to 500m, but any further out at our range and we were running into wind problems with the 75 gr tips being blown all about and hard to get consistency from it past 500 at our range. Always windy.
I sold the 223 and started to build her a 6GT. Now she has around 400 rounds down the barrel and is enjoying it. Recoil is easier for her and she can spot her shots. But it is a reloaders cartridge and factory ammo is difficult to find.
I was planning of building myself a new set up, now that she has my action. That all went out the window when I found a great deal on the EE for a 6 dasher. I wasn't planning on a 6 mm, but hard to pass up this deal. Ticked many of the boxes for what I wanted. Low round count, left hand action, single feed, bench set up. I shoot right hand, but have longed to try a left hand action off the bench for shooting right handed. I'll know in a week or so if I made the right choice or not!
While granted it will be hard to find a 6.5CM for a deal, they are out there. Me, I reload so the dasher fits well into my reloading set up with new dies, new brass, and new bullets already on the way. The other low cost bulk ammo that is decent will be for the 308. Once you shoot out that barrel you can always change up caliber with the new barrel, as long as you stay in the bolt diameter range of the 308.
When I shoot out the dasher, I can always change up into a caliber of preference.