Everybody says all asian machines are the same, but I sure havent found that to be the case- they may have been cast from the same patterns, but after that there is a real range of quality.
As a tool importer, you can order from a large menu of options- starting with the quality of the castings, the degree of finish, the quality of fasteners, electrics, and other parts, and the sourcing of motors.
I have never owned a Birmingham tool, although I have heard relatively good things about them.
But I agree that availability of parts and service is a big point- I think that Jet and Grizzly are the most reputable when it comes to that- Grizzly starts with cheaper, lower quality tools, but they stock parts in 3 locations across the US, and they are good about replacements. I have been in their main store in Bellingham, and they actually have tools and guys who know how to use them there- a motor rewinding shop, a surface grinder big enough to regrind their biggest tables saw table, etc.
Jet seems to sell higher quality stuff to begin with though- the few Grizz tools I have owned are the cause of constant bad words- in fact right now I need to rebuild the vise on a grizz bandsaw because the way it was designed and built just plain doesnt work, and never would have.
Wheras I have several Jet tools that are over 10 years old, with constant daily use, and they still work fine- granted, mostly support tools like sanders, drill presses and grinders, but I know their milling machines are made in Taiwan, not china, and I would recommend taking a look at them as well.
I know with Jet you can specify made in US motors and pay a little more on many of their tools. And I have had them send me parts for free several times, once on a 20 year old tool.
This is not to say you shouldnt also look at used american made tools- and you know, you can get a brand new bridgeport from MSC for something like 12 grand- only 2 to 3 times the price of a grizz, jet or birmingham.