Kinda cute you call your straw man "Stark"...For trajectory purposes I prefer to hunt with something that has a muzzle velocity near 3000fps or more. I know everyone says “rangefinder and dial your scope”, but whether you’re dialing or holding, and whether you’re using a range finder or your eyes, there is error in all of it. The flatter your trajectory less sensitive your shot is to all of those errors. If you actually compare a .308Win with a 180gr Partition to a 26Nosler with a 140gr Elite Hunter the difference is stark.
.308Win 180 Partition 2600fps
100yd zerp
480yds -57.82”. -47.09”
500yds -64.47”. -53.30”
520yds -71.58”. -59.
26Nosler 140 Elite Hunter 3300fps
100yd zero. 200yd zero
480yds. -29.97”. -25.11”
500yds. -33.42”. -28.36
520yds. -37.08”. -31.82”
So a 20yd ranging error is resulting in a 7” difference in POI with the .308Win compared to a 3.5” difference with the 26Nosler. Also, you literally had to dial half as much with the 26Nosler, resulting in less chance for error. You compound that with the error in angle calculated by your range finder, the fact that a lot trajectory calculating range finders don’t use your actual BC, but just one that’s close, the fact that most manufacturers BCs, especially G1 BCs are fairly incorrect, the fact that your “custom dial” was set for a certain elevation and temperature that you’re not actually hunting at etc. and all this range and dial stuff isn’t as fool proof as we pretend it is, and shooting a cartridge/bullet combination with a flat trajectory is as valuable today as it ever was. Yes I choose two extreme examples, but statements are FREQUENTLY made on this forum that suggest that as long as you have a range finder and a custom turret you can make perfect hits at any range with any cartridge regardless of trajectory. It’s flat wrong. It works ok at the range because the range is always at the same elevation and the targets are usually at the exact same range and angle. Heck, I don’t have as good a hold on an animal as I do at the range. Why would I want to introduce other problems I could avoid? My personal preference is to get as close to 3000fps as I can.
Now I will finally answer your question.
A 6.5mm 140gr projectile is on the light side for elk, especially if you’re considering shooting one at a distance at which error will be greater. Sure, a lot of elk have been killed with a .243Win shooting a 100gr bullet, but most of them were with near perfect shots. If you’re going to be trying to do that at 500yds, You really want a bullet that will be a little more deadly in case of a less than perfect shot. A 6.5PRC under normal circumstances will push a 140gr bullet at about 2950fps, so for my preference, it’s pretty much topped out at 140gr. Compare that to a .280AI. A .280AI will push a 168gr bullet about 2950fps. In VLDs the 168gr 7mm has a higher BC than the 140vr 6.5mm. Depending on the manufacture BCs may end up about the same. If you’re going to shoot elk, why not hit it with 20% more bullet, especially since you have not sacrificed trajectory, BC, or impact velocity? You can wring a little extra from both cartridges with longer barrels, bullet coatings, and hand loading up to pressure signs. If you will accept a lower velocity, you can still only get to 156gr in the 6.5PRC, and can almost match that trajectory with a 175gr in the .280AI. If trajectory does not matter to you, you can go over 190gr in the .280AI.
I’m going elk hunting this year with a 6.5-257AI shooting 156Elite Hunters at 2975fps. If I had a .280AI(stalled build) I would carry it instead and would shoot a 175-180gr Berger around 2950fps.
I'd not let 20-28gr bullet weight factor in my decision.
To the OP, IMO the two you are looking at are as capable as each other regarding your desired uses. Therefore, I'd focus on getting the rifle I wanted. Some makes/models come in one, but not another.