In a realistic comparison between these two cartridges, I think you will find that the 25-06 is always faster, usually by about 200 FPS, than the 243. And that speed shows down range with more energy, less wind drift, and significantly flatter trajectory. Just depends if you need the extra power that the 25-06 provides. I personally like the 25-06 because of the flatter trajectory and extra energy for deer hunting.
Below is a comparison of Ballistic Tips in the 243 and 25-06 as provided by current factory loads in Federal ammo. Before any one bad mouths the ammo, I have shot Federal ammo in 4 different cartridges and various bullets weights across chronographs and found their stated velocity and drop data to be pretty accurate. It does offer a good realistic comparison when trying to compare cartridges. So..... Below is data for the 243 95 Gr. Ballistic Tip and the 25-06 100 Gr. Ballistic tip.
Bullets:
243: 95 Gr Ballistic Tip BC= 378
25-06 100 Gr. Ballistic Tip BC= 394
Case Capacities:
243: 54.8 Gr. H20
25-06: 65.8 Gr H20
The 25-06 has 11 Gr. or 20% more capacity
Velocity:
25-06: 3220 FPS
243: 3025 FPS
25-06 is 195 FPS or 6.4% faster even with 5 gr. extra weight
Drop at 600 yards:
243: 73.1"
25-06: 61.7"
25-06 has 11.4" or 15.6% less drop
Wind at 10 MPH
243: 35.1
25-06: 61.7
25-06 has 4.8" or 13.7% less wind drift
Energy
243: 613 Ft lb
25-06: 793 Ft lb
25-06 has 180 Ft lb or a whopping 29.36% more energy
I think you will find this holds true across all bullets and powders you use with these two cartridges when comparing them in like barrel lengths.
For all of those using this or that powder to get extraordinary speeds from a 243, remember that there is a powder bullet combo that will work just as well for the 25-06. And in the end the 25-06 will be about 200 FPS faster and have all the benefits that come with that speed.