It really depends on the actual individual barrel as to how much velocity difference between 24" and 26" barrels. On average a 26" will give you 100 to 150 fps more velocity depending on the powder used.
For really good store bought ammo for deer thumping with a 25-06 Federal Premium has two loads. One is loaded with the 117 Sierra Game King and the other is loaded with the 115 Nosler Partition. Both will get-er-done really well.
I also see that Winchester Supreme offers a round loaded with the Nosler 110 Accubond bullet for the 25-06. I have not used this bullet yet in my 25-06 but did use the Accubond 130 gr in my 264 Win mag this deer season to shoot 4 deer and it really did a great job. I bet it would do great in the 25-06 also.
Winchester also has a 115 gr Nosler ballistic Silvertip load that I have seen others use with great results on deer.
I hand load the 117 Sierra Pro hunter for my 25-06 for deer. My rifle does not like boat tail bullets like the Game King but will shoot lights out with the flat base Pro Hunter for some reason. I have killed some deer with the 115 Nosler Partition but they did not kill any better than the Sierra and the Sierra is MUCH cheaper. Both will drop a deer in it's tracks 99% of the time if you put one in the center of the front shoulder. Sight your rifle to hit 3 inches high at 100 yards and all you have to do is aim in the center of a deers front shoulder out to right at 350 yards and you will kill it. The bullet will not rise or fall over 4 inches from muzzle out to 350 yards.
My experience with barrel length is generally about 35-40' per inch. There are a lot of good bullet options out there. For three hundred yards, I would pick what is the most accurate of some of the previously mentioned.......Rich