Here are some thoughts on the more popular 6 and 6.5mm cartridges you see now-

260 Remington- this was all the rage a few years ago. Brass is available from a number of places, SAAMI spec is .060" freebore, for most bolt guns .080-.100" is better. Feeds well from AICS magazine.

260 AI- similar performance to the 6.5-284 but you have to fire form brass

6.5-284 Norma- This was the shiznit a few years ago for 1,000 yard. Doesn't work so well in the short actions with long bullets. Has a rep as a barrel burner

6.5 Creedmoor- note the spelling, it isn't Creedmore. Lots of factory ammo around. Shoots well. Feeds well from AICS mags or AR10 platform. Downside- Hornady brass sucks. Norma makes it but it is expensive and kinda soft

6.5x47 Lapua- similar to the Creed except it has Lapua brass and a small rifle primer, Super accurate. Easy to load for. Lapua made it to compete against the 6mmBR. Feeds well from AICS mags

6 Creedmoor- 6.5 creed necked down to 6. Less recoil, presumably less barrel life. Stuck with Hornady or Norma brass. 6 Creed brass is available without the need to neck down. GAP started selling it, other vendors followed

6x47 Lapua- 6.5x47 Lapua necked down to 6mm. Less recoil, presumably less barrel life than parent cartridge. Harder to load for than parent cartridge.

6mmBR- super accurate little 6mm. Downsides are the ability to feed from AICS magazine without some sort of blocking. Also, it is kinda short so extraction can be difficult to tune at times.

6.5 Grendel- solves problems that don't exist. The published velocities are from a 24" barrel and are really optimistic. In testing I couldn't come close with close to 50 loads. For a AR I guess it makes sense. For anything else it is silly. No, it isn't the same thing as any of the cartridges above.

243 Win- the old standby. Great little hunting cartridge. Not so great in the SAAMI config as a match cartridge. Feeds well from AICS mags. Most of the match guns have longer throats and may/may not run an improved shoulder. The 6XC is sorta a short 243