I bought a Canon 270EXII flash for my EOS-M. They make the 90EXII specifically for the M, but it's pretty limited. The 270 is a little more $$, but for the extra benefits I think it's well worth it. It has a metal foot instead of plastic and you can pivot the flash head up to bounce off a ceiling. It's also more powerful. It's slightly larger than the 90, too. But it's much, much smaller than the 430EXII. I ordered mine yesterday and got it today, courtesy of Amazon Prime.
Here's a comparison shot.
