That interesting because you can still get new 2600 series tractors.The 1600 series is built by Mitsubishi in Japan. The 2600 series is built by TYM in Korea and has been discontinued.
I might be wrong, but I believe that the 2600 series tractors that are still available are simply units that were already made. I'm pretty sure that the agreement with TYM has come to an end.That interesting because you can still get new 2600 series tractors.
One of the biggest differences is the seat and how it sits, one seems more like you ae sitting on it the other seems more like you are sitting in it. The 1640 is a nice tractor, I have the 2638. I bought it new 4 years ago, I have almost 800 hours on it and I have had 0 problems with it that I didn't create myself. I think on paper the loader capacity is higher on the 1640, operate them both and decide.