There is so little practical difference between the 6.5x55, .260 Rem and 6.5 Creed, in factory loads, that I would say, pick the rifle that best suits you and buy it in whichever 6.5mm they chamber it for.
Handloading does tend to favor the 6.5x55 and some rifles use a standard length action so you can, as you said, load heavies to their full potential.
My favorite low recoil, will handle 90% of the game I am likely to shoot, rifle is my 6.5 Grendel on a CZ 527 action. My favorite lightweight, full power rifle is my Kimber Hunter in 6.5 Creed. I like that one because, as many have already pointed out, there is a vast assortment of reasonably priced factory ammo available. Oh, and it shoots pretty darn good too.
I own and shoot 6.5's in the Grendel, .260, .6.5 Creed, 6.5x55, 6.5-06, and 6.5x65. I started both my boys at age 8 on the 6.5x55. I loaded 100 grain BT's at 2700 fps and my youngest killed a massive 250 lb PA whitetail with that load. They have since graduated on to full power loads.
If I had to choose one, I'd keep my custom 98 in 6.5x55. But I could live with any "one" of my 6.5's if I had to.
I do handload (and cast) so I have some powder and primers to start with, and will probably look to trade off some brass for whatever 6.5 I settle in on. I like to shoot, so I can see having some fun developing loads up and down the spectrum of the cartridge's capacity. Given you have such a collection of 6.5s, including the Grendel, have you found a common powder or bullets you can use across the different calibers? Is there a difference in brass life, etc.? I'm not a "box a year" shooter, especially since I've gotten out of collecting firearms, calibers and wildcatting. The fun for me now is to thoroughly work with a single firearm to our fullest potential, which requires range time and reloading IMO.
I've played a little with the Grendel in a 527 and Howa Mini - that's a fun caliber for a shooter, and probably great for most hunting in the lower 48!
