There are a lot of options and a few considerations to think about. Do you ever see yourself wanting to shoot farther for hunting or play? What types of bullets? light for speed and flatness or heavy for longer range or the option for both? Would you want to always shoot handloads, or do you want the option of picking up some factory loads if needed.

For deer at 350yds, a 6.5 caliber should work very well with pistol speeds and most hunting bullet choices. You aren't really stretching the legs of the high BC bullets you can get in 6.5 so you can launch them slower, and give up a little of the flatness for reduced recoil if needed.

For what it's worth, I looked at Mid-way's list of ammo and found 70 for the 6.5 CM, 23 for the 260 and 18 for the 6.5x55. I load my own, but I take comfort in knowing there are options at the corner store if I need.

The 6.5cm has a lot of hype that drives many away. As someone who also shoots and reloads a lot of 25-06, I feel the biggest advantage is the prevalence of high BC 6.5 bullets and fast twist barrels. With the long-range focus comes the added benefit to pistol shooters of lower terminal velocity requirements for many of the popular 6.5 bullets. For example, the Barnes 127gn LRX for the 6.5 is reported to have 2x expansion down to velocities of 1600 fps with a BC of 0.468. Most traditional hunting bullets have 1800-1900 minimum velocity requirements. Even the Nosler BTs are up in that range.