Arguments about which cartridge is better are as old as... well, cartridges.
Some argue the infinitesimally small differences between very similar cartridges (6.5CM vs 260Rem vs 6.5x55, for example), others default to the short action vs long action (308Win vs 30-06 or 300WM vs 300WSM). Most of the time, these arguments are as useless as they are entertaining. Having read through a lot of them, most of the arguments fall off as fluff and anecdotal observations, but a few things stand as something to keep in mind. A lot of this really pertains to a simple question of whether all these new cartridges are just marketing exercise or if there is really some merit to them.
1) Cartridges designed more recently, especially things that Hornady had a hand in, usually accommodate heavy for caliber long ogive bullets. Those were originally match bullets, but there are now plenty of hunting designs of similar geometry. The overall trend in recent years has been toward more aerodynamic bullet and toward solid copper designs that are long for the weight and stay together well. That also means the rifles chambered for newer cartridges will often have standard faster rifling twist rates. With modern bullets, when in doubt, it is better to err on the side of a heavier bullet and faster twist rate.
2) Cartridges designed more recently are usually standardized at higher pressures, so factory ammo will get you faster muzzle velocities. There is also the problem of many US ammunition makers loading older cartridges at lower pressures because in our litigious society they are afraid that someone is going to pop this round into a rusty World War I relic of a gun and blow himself up. There is no consistency to this, but a lot of ammo made for older calibers like 6.5x55 Swede, 8x57, etc can be subject to this phenomenon. European companies often ignore that and load them to CIP standards (that are often slightly higher than SAAMI), but US companies will often stay well below SAAMI. If you shoot factory ammo, this can make a difference. If you are a handloader, you can do what you want, but it might mean going above the powder charge in the reloading manual. A good example here is 6.5CM vs 260Rem vs 6.5x55. 260Rem is a pretty temperamental cartridge to handload for, so I am not fan, but 6.5CM and 6.5x55 are very forgiving. If you handload for a modern gun and to the same pressure, 6.5x55 will handily outperform the 6.5Creed since it has a larger powder capacity. It will need a longer action. 6.5x55 is one of those intermediate cartridges and is probably one of the main reasons Tikka T3 action is designed the length it is. If you are a Tikka shooter, there is only one action length, so there is no weight or length advantage to shorter cartridges.
3) Longevity for many newer cartridges is questionable and depends on how well supported they are by the factory. 6.5CM is clearly here to stay because Hornady is very much behind it. It is not the case for all of them.
4) The fad of short and fat cartridges is somewhat fading, but there is always an argument about cartridge efficiency and things like that. There appears to be some truth to it, but not as much as people think. The counter argument in favor of longer slimmer case design is feeding reliability. Most recent designs seem to have found a reasonable compromise, but there is a reason WSSM cartridges are essentially dead as are most of WSM and RSAUM ones.
With all that said and done, each rifle is a law onto itself. Each barrel is different. One thing that is certain is that the quality of modern bullets have made lethality differences blur somewhat. Proliferation of laser range finders and ballistic calculators made trajectory compensation a lot more straightforward than it used to be.
I err on the side of calibers with mild-ish recoil that can be loaded with a good bullet that is either a little heavy for caliber or expanding copper for hunting. I want to retain the ability to spot my own shots whenever possible, whether practicing or hunting. Practicing is key. 8.6 Blackout is becoming my primary hunting round in both subsonic and supersonic guises, but it is not exactly flat shooting. That's a compromise I am willing to make for increased lethality of a fast twist barrel, handiness of the overall package and mild recoil. It is not the right option for everyone, but if you put in the time, it works great.
For something a little bigger, I have a 300WSM. Funny thing is that now that I have a pretty good idea of what I might need, it is really not clear whether I need anything between the 300WSM and 8.6Blackout.
My kids are both saying they want to go hunting with me and they will start with flatter shooting rounds until they can make their own decision on what works for them. That's why I am keeping the 308Win and the 280Rem. I could easily do 30,000 words on why a 6.5CM is better in this case than the 308 and 7mm-08 is better than the 280Rem. I could also do 30,000 words on the opposite case and another 30,000 either way on 308Win vs 280Rem. All of those differences are in the noise as long as I use a good bullet, make sure the stock fits properly and practice.
