I don't own a 25/06 but do own a .243.
From my own experience I know the .243 is fine for deer as long as the hunter/shooter respects its use and takes appropriate shots. The 25/06 I only know about from reading and seeing other peoples successes. from my armchair they are both fine cartridges. If I didn't own a .243 and a bunch of 6.5 X 55's I'd probably buy a 25/06.