Unless they've gotten a hell of a lot better since I was last looking at them (a year or so ago).

This depends a bit on what you're looking for, though. The core M is ultra-low-voltage, so it'll be more battery friendly and less hot, but there's a price in overall horsepower. If you're basically looking for a tablet with a keyboard, then the core M is probably a plus. But if you're planning on doing anything more intensive, then I'd lean toward the i3/i5/i7. These are probably getting rarer in ultraportables, though.