Well the .243win is one of the most popular rounds in the world but that is not just because of the .308win parent case but because of the bullets you can launch that is the 6mm department.
So the .243win is hard to beat but you have to run it on an AR-10. DPMS makes a good one but I heard better go for the shilen barrels.
The 6x45 carries plenty of punch and feels the void quite nicely.
It can put a 90gr bullet with very good BC at 2700fps and 2750 with a hot hunting round. Not that is needed but plenty to take care of a white tail out to 300 or even close to 400 yards and a coyote at 500 even 600 yards but at those ranges the least of your worries is the system you choose and the gear and the training is where the big bucks go. Most folks are not ready to do that.
One thing is sure, the 6mm bullets can take care of many types of game not just white tail.
I feel that for many uses the 6x45 will work just fine, like in places for an inexplicable reason the .223rem you cannot hunt, but it is up to you if you need the extra punch of the .243Win that is a very nice round but need the extra meat to carry around with the AR10.
Both can be extremely accurate and capable of below 1/2MOA if you put the dollars in a match barrel and some reloads.
The 6x45 brass is virtually free at the range bin so something to take into account.
The lothar walther I got is a very good barrel and is giving below 1/4MOA already.
I liked the 6x45 project because of the simplicity. That is why I like the .223 REM high grain rounds and all the other rounds that do not need extra mags, oversized bolts and whole nine yards. It keeps the systems better integrated, I think.
I have around 140 AR-15 magazines and only a few dozen among all the other rounds so you cannot beat standardization.
I still believe our troops should have the 6x45 with 80gr to 90gr sniper rounds.
There will be less need for the .308 in many situations the 5.56 cannot deliver the mail.
Read this....
http://www.chuckhawks.com/6mm_military_cartridge.htm
I you consider 6x45 you can buy this one but I think the one you build could be as good or better. Also plan in reloading, black hills is expensive
and do not shoot that great anyway. I went with a 1:9 rate and it has no issues stabilizing the 100grainers.
http://www.lesbaer.com/6x45.html
I hope this helps.
Cheers.
E.