Up until a year or so ago, the 15 series included the 2015, 2615 and 3015 (each available in gear or HST). Last year, they added four more to the mix 1815, 2415, 2815 and 3215. So, the 18, 24, 28, and 32 are a newer design. Some folks like the new design better, some not so much. I haven't really looked at the newer ones that much, so I don't know what the real differences are.
I really like my 2615 4WD (gear). The only thing I wish were different about the basic design is the location of the loader joystick. Since I got my grapple, I'm using the loader almost continuously. Having to reach for the stick constantly gets tiring. I may attempt a modification one of these days. I think I could cut the "post" that the control valve and joystick are mounted on and weld it back adding a little angle to it. That'd put the joystick in a more comfortable position, but would eliminate any chance of mounting/dismounting the tractor on that side. I always mount/dismount on the left side anyway.
I think the newer models in the 15 series relocate the joystick to the right fender, eliminating this problem.
Later,
BR