Well, if I have my ducks in a row it's like this: 936D is offset 1' to the right leaving 17' on the left, and 19' on the right.
635D, being centered will give you 17 1/2 on the left, same on the right.
Thus, you will lose 6" on your new rig.
My understanding is that Deere came with the 26' auger to 'fit' with their new 40' head.
Personally, I would get the longer auger in a heartbeat, you can never have too much room. Perhaps you will go to wider headers in the future, and JD, Honeybee, and MacDon are all centered now I believe.
Guess you always have the option of putting the Stewart Steel extender on, but surely factory is better? I can't imagine that the longer auger wouldn't give you a little better resale; perhaps the 'next guy' will want the longer auger for a wider head.
Seems like bigger is better in most things. Just my .02.
Just noticed--you mentioned unloading in trucks, guess room wouldn't be quite as crucial as into a moving cart. Your call.
NO problem with the trucks but our grain cart is 14 feet wide with tires and that is more critical. ORdered the longer auger for next year. Had no problems this year but the combine drivers were a little nervous with the cart operator at times. Not much room for driving crooked when you are trying to fill the cart right full on the last dump.
Had lots of room with the 15 foot pickup headers though