Oh, the fun you'll have.
Several years ago I bought a micRo M3 from the now-defunct LumenLabs. I got one of the last few out the door before they folded up their tent and faded off into the night. It's a leadscrew-driven machine that uses a flex shaft tool for the spindle. I've beaten the you-know-what out of it milling mostly ABS plastic (cases for ham radio kits) with the occasional wood, HDPE, aluminum and a fingernail or two. It's still ticking along like a Swiss watch, which is good since the parts are not available anywhere if it ever breaks. In theory I could make all the parts for it, but the labor and precision required is apparently what did the company in to begin with. Fortunately it seems to be nearly bulletproof. Here's what I have learned...
Buy the biggest machine you can, and if you run out of machine it will probably be the Z axis that you wish you had more of first. Right now I'm wishing I had a 4' x 8' Shopbot, but that's just because I'm doing some woodworking.
Don't buy a cheap POS driver board. Those 5-axis Chinese eBay specials? They don't even make good paperweights. A Geckodrive G540 is worth every penny you pay for it.
If you don't need to mill wood or metal, a 3D printer may be enough. I need the CNC machine for my kit business, but a 3D printer would get a lot more use for my own stuff if I had one.
A Dremel or Foredom/WeCheer flex shaft tool is OK for plastic, marginal for wood and barely adequate for aluminum. You will not regret going with a more powerful spindle motor. And you will need a speed control no matter what you choose.
I haven't tried any of the machines like the ones you're looking at. What I would be most interested in is the repeatability of the thing... if I move the tool to all four corners ten times, will it return to the exact same spot EVERY time? Within what tolerance? I see some crazy accuracy claimed on some machines, but the real question is how much mechanical slop and backlash is present, or will be after a few hours of use.
Wish I had more specific and current help to offer. I like the leadscrew drive like the Stepcraft, though I would ask them about what those leadscrews drive and how the backlash is taken out of the system. I haven't seen one of the belt drive machines up close. A leadscrew machine can have better motion resolution, a belt drive machine could move a lot faster. I wouldn't run away from either one of those, especially with a decent spindle.