in my past expirence I dont think there will be much fuel savings. You will be trying to do the same amount of work and it take an amount of fuel to get that done. Yes the bigger tractor may take a little more but they both are pulling the same load across the same distance so it will be really close to the same consumption. I have even noticed that with a larger tractor you tend to get the work done faster using more fuel per hour but it comes out to the same per acre when all is done.
Another way to look at it would be to get the 105 so you dont put the wear and tear on the 135 because as we know they are not getting any newer and parts get harder to find everyday and then you have the possibility of a second tractor if something happens to the other and still can get the work done when it needs to be done, but I dont think the fuel savings will be a big difference.