I always think you should start out learning 8-bit micros like PIC or AVR. The datasheets are short enough you can conceivably read the whole thing. This gets you some instinct on what peripherals a micro should have, how to look up their functionality, how to troubleshoot when they don't work, etc.

There's a much higher bar for getting everything working on an ARM controller (ie: did you turn on the peripheral clock before trying to drive that pin?). But I agree, once you're initiated you should prototype everything on a 32-bit micro and then port to smaller or more cost effective chips once all the functionality is established.