I was looking at a 1600 series before the dealer recommended I take a closer look at the 2600 tractors since my farming operation was starting to expand and my current 1533 I have almost outgrown. In short, the biggest difference is that the 2600 are more entry level ag/farming tractors whereas the 1500/1600 are still compact utility class and not as forgiving when it comes to doing work using ground engaging implements. Not to say you can't do these things with them, just that the 2600 is built more like an ag tractor, for real farming work. The configuration is more like a old-school tractor, with the 8x2 shifting in the center of the driveline between your feet, shuttle shift is an option which is standard on the compacts. Same thing with live PTO.
The 2635 is a real beast though, with DL250 loader it weighs 7000lb and can lift 2900 lb. With this tractor I should be able to do ALOT more then I can currently, ie plowing, seeding, run a small square baler, not get stuck in the mud...
Dealer said that they move about 3-4 of the 2600's a year and naturally a lot more of the 1500/1600. But those who had them were happy.
Other reason why I was going to the 1600 is the subframe mounted backhoe, but I can still get it for the 2600 it's just a 3 point mount. With this tractor I'm not too concerned about anything breaking.
As for negatives, I'm not sure what they are yet, I need to learn more about it.