The 243 cannot shoot heavies, I think 105's is about max. They make 6.5 bullets all the way down to 85's. A 243 is no slouch on deer. Only after the internet was invented did deer require short mags to kill.
I love both rounds so hard to choose. If you think you will hunt bigger stuff than deer, the 6.5 makes sense. If you think you might want a second varmint gun, go with a 243 since it can shoot 55's.
I have the a few 223's and inherited a nice little 22 hornet so I am good in that area for my occasional woods coyote hunt.
That darn 6.5 is just a nice gun when you really drill down into balistics. Never understood why it suffers other than its not an american 30 cal.
