Re: 6.5 Creedmore VS 6.5x284
I'd settle this by deciding which bullet you want to shoot and your max range that you would want. Set a minimum level of energy you're comfortable with (I've seen anywhere from 600 ft/lbs to 1400ft/lbs) to make a clean kill. Then go to a ballistics calculator and figure out how much velocity you need to get at the muzzle to stay above your energy threshold at max range. Check out a couple load manuals for safe data and see which will give you the velocity performance you need. Most load manuals give velocities in 22" or 24" barrels so keep in mind you can add anywhere from 25-150 fps per inch (within reason) going with a longer barrel. Also keep in mind that this is all educated guesswork and some guns and barrels are just faster, slower, and more pressure sensitive than others.
Example, I want to shoot the 140g A-Max, max distance 800 yds and I feel good about 700 ft/lbs taking a coyote or small deer (number for example only and I have nothing to back that up, spend some time and research this)
The 140 has a BC of 0.585 and we'll use sea level for calculations just because.
Using guess and check with the handloads.com ballistic calculator, a muzzle velocity of 2700 fps gets me 823 ft/lbs at 800 yds with this bullet.
The online IMR data shows the 6.5 Creedmore at 2500-2600 fps at max loads for the 140 A-Max. The 6.5x284 comes up with 2700-2900 for the 142 gr sierra hpbt (not necessarily interchangeable for load data but good enough for our analysis here)
Based on this, I go with the 6.5x284 to feel good about my shots without having to hot-rod the loads. I've also got a good margin for error in case I end up with a slow barrel. Obviously this could get as complicated and time consuming as you want it as changing one variable means going back to the start, but it gives you a quantified comparison instead of, or to go with, a gut feeling or preference. Also remember "garbage in - garbage out" and the number at the end is only as good as the information going in.