I've got a 3040 and a 6040, the 6040 is terrific value and quite a robust and well made machine generally, the only real downside is the garbage "blue box" controller they come standard with. Replace that with a Gecko G540 and you won't believe the difference. The 3040 still represents some value but it's definitely a lower powered machine with quite a small working area.

The 3040 usually has unsupported round rails, for drivescrews I've seen some machines have an ACME or trapezoidal thread, while some have a rolled ballscrew. The bearings are usually just round linear bearings inserted into the frame/gantry as opposed to the pillow block bearings and supported round linear rails used by most larger machines. A lot of the 3040's also come with a really crappy DC brushed spindle, if you can even call it a spindle, whereas most 6040's come with an 800w/1.5kw spindle/VFD. They also sometimes come with NEMA17 steppers, while others come with NEMA24 steppers, but they are all fairly low on the torque and not much chop for any grunty work. 3040's also usually come with a drive controller that maxes out at 36V, although most are supplied with a 24VDC power supply anyway, so this also greatly limits the torque.

Having said all that though, they are dirt cheap and more than adequate for small general woodworking jobs, PCB, engraving, etc. But I think the 6040 for not much more is a far better option.

But horses for courses.

cheers, Ian