I'm a spec nerd, there are a lot of intangibles that you can not determine without actual fondle time on a machine, smoothness of operation, rigidity, ergonomics etc, but you can find a lot of differences just looking at specs and make some assumptions about some of the intangibles, specifically rigidity as weight is directly tied to taming vibration.
Comparing the PM 12x36, to 13x40 to 14x40, the difference in actual capacity is minimal, all three can work a piece of roughly the same size until you are pushing the limits and even then you are only looking at about 1" in diameter and a couple inches in length. A 14x40 may be happier turning a 6x24" piece of material (192lbs), but a 12x36" is fully capable of that. How often is a home user turning material of a size to really make a difference between a 12x36 and 14x40 lathe?
Spindle bore is probably a much bigger feature in home shop sized lathes. 3/4-1" in the 9-10" class, 1-3/8 to 1-9/16" in the 11-13" sizes. and often little difference between modern 12" and 13" lathes. The PM 14x40s have a spindle bore of 2" so quite a bit more than the PM 12 and 13" lathes.
Power, 12" are typically 1-1/2 to 2hp, 13" are mostly 2hp and 14" are mostly 3hp so there is a difference, but often not huge from step to step and there can be overlap from one to the next.
Weight (lathe only)
1022 is 360lbs
It might help to know what hobbies you enjoy and whether you are a gun guy or live on a farm or in the suburbs, etc. If you don't know what you're going to do with the lathe then choosing the right one is going to be difficult. As you can see, there is the "buy the biggest you can fit/afford" school of thought and it is probably the prevailing one on the forum. Then there is the "get the right tool for the job(s) at hand" group, of which I am one.
My suggestion is to put a lot of thought into what you think you will need to do with the lathe, then buy the best size and quality you can afford to suit your needs. You should also know which features matter on a lathe and make sure the one you choose has those features.
Once you home in on one or two lathes, ask the guys who own those lathes for feedback and they'll give it to you. The reason this matters is that you have to live with a lathe to really know the lathe. Spec sheets, pictures and hype do not give you the real stuff but the guy who owns one will tell you exactly what you need to know.
1236 is 960lbs
1236GT 850lbs
1340GT 1100lbs
the various 1440s are 1800lbs
Probably safe to assume the 14" class is a lot more rigid than a 12 or 13" lathe.