Do yourself a huge Favor and at least read around the EMC2 forum. It is Truely the better tool. I look at it like this: Mach is like BS2, gets you up and started, but then you are finished. Thats an important thing, but I like to have a bit more power.

I am fairly certain Mach can not run my 6 axis machine, but I could be wrong. With emc2 I can slave drives and run them independently at will.

As for the modular approach, I started out that way, but when I was about to add a third parallel port just to get 17 more I/O, I decided it was time to use the port as it should be, to pass bytes of data, not single pins of I/O. Then you start to think about comms, how to interface all these modules? The updates are coming every 30uS to be able to reach 1500ipm with my setup (1000 steps per inch). So not a lot of time for talking to each module.

Gecko is hands down top shelf drives. but they themselves will tell you in low current (<4 amp) steppers, they are not much better then a linear drive. But I will never try to talk anyone out of them, they are and deserve to be called - TOP SHELF! On the other hand, in my 6 axis machine, there are actually 7 steppers, 1 axis is dual driven. so thats a lot of cash for the geckos when I can build a unipolor drive that is way happy to run from my ATX 12V power supply for ~ $15 USD each. So my entire bank of drives is the same as 1 gecko.