I don't think it's fair for me to say which machines are better or worse than others. All I can give is my opinion about what's important and suggest some options you haven't looked at. Since I don't do PCB work, my opinions probably aren't all that helpful to you. It sounds like you're pretty close to deciding on the Zenbot machine. If you're looking for someone to talk you out of that or affirm your decision I'm the wrong person. In the end, buy what you feel comfortable with just make sure the vendor can support their products.
Hopefully someone else will jump in with other opinions. (Any one else out there tempted to jump in and criticize my oversimplified explanations please apply your energy more directly to help hexdump rather than being an '41 73 73 68 6f 6c 65')
If I were buying a pre-built machine I'd want to know more about the linear bearings and drive system.
It's unusual to use a belt drive on such a small machine; be sure you're satisfied that the minimum addressability is adequate for your application.
Stepper motors typically have 200 steps per revolution, meaning they can be positioned every 1.8 degrees of rotation. This positioning can be interpolated or smoothed using a micro stepping driver board, again with trade offs. The drive system the motors connect to use those steps to move the cutting tool (and/or the table) There are many different designs for drives. None are absolutely 'good' or 'bad', they each have benefits and limitations.
Machines this small more commonly use drive screws which provide a built-in gear reduction. A 10 thread-per-inch drive screw will move the tool an inch when the motor turns 10 times. Even using full steps, this gives 2000 steps per inch, resulting in a high degree of addressability (and additional torque) by trading off fast movement. A belt drive works differently, trading off some addressability for fast movements. Depending on the belt drive design you may end up with fewer than 200 steps per inch reducing the theoretical ability to position the tool at an exact location. Then again, for some applications this could be adequate. If your application doesn't require high addressability then a belt drive may be a good solution, particularly if it's well designed.
Note that addressability isn't the same as accuracy, the two are complementary properties. Accuracy is the result of the design, materials used, manufacturing processes, and many other factors.
Speaking of design, the X&Y bearing design isn't discussed in any detail on the web site. From the little I've read (
this,
this and
this ) the previous design used hardened ways and nylon or HDPE bearings on X&Y. The new design apparently uses ball bearings on square steel tubing. Does this new design have any provision for adjusting the bearings?