I have experience with three cards. The CycloDS Eve, the Supercard DS-One, and the DSTT. The TT is great for a small price, with few functions (very good for people that just don't want to pay for games anymore, and wont do TONS with roms or homebrew).
The SC-DSone is great for stability and price (lots of options, runs everything well, supports folders!! and has tons of extra options). It was my first card, and now its the wife's and she LOVES it.
The CycloDS is for hardcore users. So many functions, VERY sturdy build (of both the card, and the microSD reader, which is solid a hell!), and, major importance to me, English support... and the dev team READS THE FORUMS for troubleshooting and new ideas.
I haven't used the M3, but i'd sure give it a shot... but if you are looking for using your card for things other than casual DS playing (i.e. lots of homebrew, game switching, major use stuff) then i'd suggest the Cyclo. Otherwise, i don't see why the M3 wouldn't do you well, for having a lower price.
Hope that all helps out somewhat