There is a lot of people who subscribe to the belief that quickest is not always best, but personally I think it is a bit more than speed alone. I shot this Sika Hind with a 243 100 gr Speer HPBT, I was only about 60 yards away.
In classic accuracy style I missed the heart completely and shot it through both lungs. Well, she took off like a startled deer, which of course she was, there was pink frothy blood on the ground so we knew she was well hit, and she had just not realised she was dead yet. About 150 yards away we found her, dead, when we come to the gralloch I retrieved her lungs and both had an entry wound and an exit wound, very neat, very precise the bullet had exited the other side and there was virtually no damage to the carcase. The bullet had not appeared to open up at all, judging by the internal damage there had been no mushrooming at all. Was the round too fast to get it's work done, would a longer distance to the target have allowed it to slow down to open up? was it a rogue round. I don't know, what I do know is that since then I have backed off on the speed for my 243 on my deer rounds. I have no clue to the speeds never did chrony them, just backed off the powder. To be fair I have never had one go that far before with that round in 243.
This one I shot at about 90 yards with a 6.5X55 129 gr hornady Interlock at a range of 85 yards. Sorry about the poor photo quality.
The round hit the deer and where it's back was it's feet appeared, it really knocked it upside down, it never went anywhere. Hit it in the heart, the result looked as if the heart had been through a liquidiser, not much other damage at all really, the round never exited but, I never found it. That one had to go to the game dealer as we were on a culling mission for someone else.
So is a 243 better or worse than a 6.5? well you must make your own mind up. To me they both do the job, but if I could only have one then it would be the 6.5, the long bullet, with it's high sectional density driven at sensible speeds, penetrates and performs better than it should. It punches above it's weight.
John