I think the m3 is a pretty good way to go. (Might be because I own one, so bias exists in this post.)
I have not had any problems with it.
From the extensive research I did on the other options (I was looking at the exact same ones you were.) I found that:
g6 had some reported problems. (Some of which really sucked. I cannot recall offhand what they were, but they were serious problems, firmware corruption or something. ...extensive research, eh?) They did not happen to everyone, but they were definitely problems I wanted to avoid. But the *MAIN REASON* why I did not go g6 was the limited amount of storage. (512mb was not enough for me. I play movies and experiment with homebrew applications as well.)
supercard- initially, the price and homebrew were nice, but homebrew took a hit when a major homebrew player stopped production on a major homebrew application. (But, I do not know what the current status of this is.) supercard is still pretty cool though. Lacks a bit of GBA support, which is the MAIN problem for me. A plus for supercard is its saving features, which I envy. (M3 does not have horrible saving features. In fact, it does not dramatically change the way you do anything. But it definitely affects me psychologically.)
Anyway, reason why I chose the m3 (microsd by the way) and some factors that played a role in the decision:
1. I like games, and I hate carrying things around. M3 has good support for DS and GBA. (game support, and I do not have to carry my game carts around. especially gba carts, which are an annoying size in my opinion. They are durable, but hover between that state of "durable-ness" and "not-quite-durable-enough-to-survive-a-roadtrip." NDS cart simply cannot survive. ...my NDS case is a Q-tip box with socks padding. () )
2. Supposedly, M3 has a slow menu system. (When compared to the g6 touch menu, or supercards... something. I think it scrolls faster? Cannot remember.) Anyway, menu scrolling/whatever does not mean anything to me. I spend like... 5 seconds in the menu anyway. Do not get me wrong, a touch screen would be "cool", but frankly, I do not care all that much. Again, I play games, not navigate through menus. But, if you are trying to impress your friends, or are big into asthetics, then you might want to factor this in. It does have some slowness to it.
3. M3 products have been kicking a lot of ass for a while, and receiving a lot of support. So, me being a consumer that requires a lot of customer feedback before making a purchase, M3 seemed to be good. (Note: I am not saying the other cards have not been kicking ass. It was a very difficult choice between the 3.)
4. External storage device. It is a necessity for me.
,,,but ultimately, I went with m3, and it has performed great. No problems at all. Right now I have Pirates of the Caribbean on it (The movie) and 12 DS games or so. No problems at all.
One complaint I do have however (about the m3microsd) is the microsd itself. That thing is so freaking small. I feel like I am going to break it every time I touch it. ...this is more of a complaint about micro sd, than the m3. The m3 actually houses the micro sd very well. It is spring loaded and everything. But make sure you work in a windless environment, because those micro sd's will seriously blow away.
Edit: I had the word "though" in like, 4 consecutive sentences. Made me look like a bit more of a dumbass than I can actually take credit for.