Albert,
I have built a couple of each and the 338-300 Ultra will give you about 75 fps in a 26" tube with either bullet. I have noticed that the 338-300 has an advantage with the heavier bullets and with longer barrels if you combine both the longer barrel and heavier bullet you see a pretty fair advantage in the 338-300, I've seen nearly 150 fps difference with a 30" tube and 300 SMK's all other things being equil.