Canada, I understand your biased towards the 25-06 as I am to the .243.
With a premium bullet compared to an average one in the -06 the differences are minute. Heres a comparo of the two, you will notice the .243 has 200 ft more velocity but has 200 ft-lbs less energy at 300 yds. it has a better coeficciency and is a little flatter shooting, now again this is premium compared to one that is not.
But back to the question I would take the .243 over the .223 for sure it has a lot more power, not to say the .223 wont kill deer its just that the .243 is just a better round all around. But you couldnt go wrong with either the .243 the 6mm or the 25-06, these in my opinion are the best cartridges for whitetail.
243-25-06 comparo