The 90EX has very little appeal, to me. Yes, it's tiny, and that's good. But it can't be bounced, meaning its only use is fill flash, and it's on the weak side even for that. Optical master? Great feature, and a cheap way to get it! But my flashes are 600's and I have an ST-E3. So personally, I think the 270EX II with more power and bounce capability is a much better choice for the EOS M.