AC 170-175-180-185

Do I have this right?  The 175 in just a newer version of the 170 and the 185 is just a newer version of the 180.  What are the real life differences and which is a better tractor?

Yes, the "5" tractors were simply sped up versions of the "0" tractors. For instance, the rated load rpm of a 180 is 2000, and 64 pto hp, the 185 is rated at 2200 and develops 74 pto hp, while fuel setting is only slightly higher. The marketing strategy was to pull the same smaller equipment faster than the previous tractor you used, thus saving drivetrains. As speed increases, the torque falls, torque is what does the work, but also damage. By the time these tractors came out, a 75hp tractor was no longer the big horse anymore either, in many cases. 

Without digging spec sheets out, ones without advance are the same whether 180 or 185, the ones with advance are same as each other as well going odd memory. It seems they have 5* pump advance.

26* on non advance, 180 w/advance is 22*, 185 with advance is 24* according to a sheet I have for all sorts of AC applications. They would probably start better at 18* lol! The 200 tractor underwent many changes, first pumps were 5* and timed at 24*, then went to 8* pump advance to time engine to 18* for better starting.