So, reading this thread, and having participated in dozens of similar ones I just want to point out a few things. You talk about classes of lathes and I think that's a good analogy, kinda like what class boxers fall into. There are lightweights, middle weights and heavyweights and many more divisions than I know.
The most important thing I see here is weight of the machine in question. Someone on here said once "you can't add iron" What that means to me is you can add accessories, tooling, DRO's, and all sorts of things to make your machine more useful but you're stuck with the base castings your machine came with. Since you have a 6x18 you already know the limitations of a small machine so we don't need to explain that.
Just looking at the specs from PM the 12x28 weighs in at 490lbs without stand, while the 12x36 is 1200lbs with the stand. That's a big difference that will impact the cuts you'll be able to make with each machine, kinda like putting a bantam weight fighter against a middleweight. The technique can be great for either but in the end the middle weight will win most of the time.
Speaking of stands....
You mentioned using a toolbox on wheels for your stand. While that may work I suspect you won't be happy with it in the long run, toolboxes just aren't built to take that kind of weight on top and when you add tooling it's easy to exceed the overall capacity. Remember this a dynamic load, not static like a drawer full of tools.
What I'm planning on is building a tube steel stand with space to accommodate toolboxes. The stand itself will have both wheels and leveling feet, I have the cheap Chinese knock-off leveling feet but honestly I think cast iron wheels with separate leveling feet will be the way to go. I also want to build fine lathe leveling into the top of the stand so I don't have to lay on the ground so much.
@davidpbest has a great write up on the stand he built for his PM 1340, also many other projects including how to make a solid plinth to increase the rigidity of any lathe.
So for me I'd say you're really looking at two classes of machines between the ones you mentioned above. Taiwan builds better machines but precision mostly depends on how you use the tools, at least at the hobby level. For the extra $600 in my mind it's an easy decision, mass wins almost every time when it comes to machine tools. You may never need it for your application but if you do you'll be happy it's there.
Cheers,
John