Since the OP is talking deer hunting, either will work very well. I look at it this way if you want ammunition availability when you walk into any store that sells ammunition the .243 is a better choice. If you are happy to rely on buying ammunition online or reloading then the 6.5 Creedmoor has a lot to offer. I have a few .243 rifles and no Creedmoor in the safe. I wouldn't hesitate to get the Creedmoor but it doesn't do anything my other rifles don't for hunting.
Now IMO there has been a lot of misinformation on the 6.5 Creedmoor that is perpetuated by the internet as well as here on this forum. Some obvious ones that stick out to me is jmr40's comments. Yes the 147 vs. 150 grain argument is correct, but is that a fair comparison because the bullets are close in weight? A more fair comparison would be to compare the 147 6.5 bullets vs 200 grain .308 bullets, because they would be of similar ballistic coefficients. If you want to use a bullet with a .697 BC vs. a .415 BC bullet and call it a similar comparison I guess you can.
His second argument (6.5 CM vs. .270 Win) only works is you are only using Hornady Superformance ammunition which has a 129 grain Interbond or SST at 2950 fps. That is close to the standard factory load of 3060 fps for the .270 Win, and the slightly better BC of the 6.5 bullet does take over at 300 yards. However if you compare the standard load of the 129 grain 6.5 Creedmoor which is 2820 fps, it can't catch up to the .270 Win until you're past the 500 yard mark. The same thing happens if you compare the .270 Win Superformance load of 3200 fps to the Creedmoor's Superformance load.
Now lets talk twist rates of the .243 Win. The majority of manufactures use a 1:10 twist rate, regardless of what Mr O'Heir likes to post about the 1:9.25 being the most common. The 1:10 twist will stabilize the majority of 100 grain bullets and factory ammunition designed for hunting. The main exception will be Berger bullets or ammunition using Berger bullets that will require a faster twist rate. I do think you'll eventually see an update in twist rate on .243 rifles with a trend towards a faster twist because the bullet are only getting longer.
Current production rifles that use a faster than 1:10 twist rate:
Barrett & Legendary Arms Works 1:8
CVA, Montana Rifle Company, & Ruger 1:9 twist
Remington 1:9.125
Savage 1:9.25
Current production rifles that use a 1:10 twist rate:
Bergara, Blaser, Browning, CZ, Henry Long Ranger, Howa, Kimber, Mossberg & Sons, Nosler, Rossi, Sako, Styer, Tikka, Thompson Center, Weatherby, and Winchester
As you can see there are far more manufactures using the common 1:10 twist vs. the faster twist rate. However, in Mr. O'Heir's defense the rifles (Remington, Ruger, & Savage) with a faster than 1:10 twist are the most talked about on this forum.
The older I get the more I realize the best hunting rifle is the one you already own. Learn the capabilities of what you already have and use that rifle to the maximum potential and you'll be a successful hunter. You'll also have more money in your pocket that'll give you more opportunities to hunt outside your normal areas. I've spent a lot of time and money chasing the perfect big game hunting rifle and if I'd just stuck to the .270 Win when I bought it 22 years ago I'd have been better off. The problem was the .270 Win worked boringly well, killing everything efficiently that I pointed it at.