They are practically two peas in a pod. I'd go with whichever chambering in which you can find a suitable rifle. I believe the 6.5CM currently has the edge for rifle and ammo offerings/availability.
In any event, I'd start with some really light loads, like Blue Dot or Trail Boss and some plinker bullets. My kids liked the 260 and 7mm-08 with those powders and light Sierra HP bullets. After that, they graduated to H4198 loads with hunting bullets.
^^this...
I have both and they both work very well for what you are wanting to do
