"Most users are going to 64 bit...."?? On what do you base this?
Probably the fact that 99% of new PC's come with a 64bit OS.
The biggest issue with motion controllers is support of the plugin. Because Mach3 is so complex, I'd be willing to bet that no plugins support all of Mach3's features 100%. Most users will probably never notice this, though. The question is how good are the developers at fixing issues when bugs arise.
From what I read, I'd say that the Smoothstepper is the only device with what you might consider a "reasonable user base". It's hands down the most popular by a huge margin.
I/O ports should have proper connectors and be reasonably buffered
Not sure what you mean by proper connectors, but most of these devices still require the use of breakout boards, often connected with ribbon cables.