Uhmmmm, If these arrows are both being shot out of the same bow I think one of your numbers is way off.

If you are shooting a 400 grain arrow at 260 fps then a 260 grain arrow is going to be traveling a heck of a lot faster than 280 fps. More like 320 fps. A bow that is shooting a 260 grain arrow at 280 fps is not going to be able to shoot a 400 grain arrow at 260 fps. If you are useing IBO 5 grains per pound then the 260 grain arrow is being shot out of a maximim poundage of 52 pounds. I don't know of a 52 pound bow that is going to shoot a 400 grain arrow at 260 fps.

Could the 260 grain arrow actually be a 360 grain arrow?

If that's the case, then for whitetails, I would use the arrow that shoots better. If you are hunting bigger game than deer then use the heavier arrow.