I just posted my thoughts on this subject in another thread, but I'll post a couple here as well.
First of all, I can't say anything about the corruption issues. I have no idea why they occur, but I have a feeling its the contacts not always touching the way they should. What I mean by this is the G6 lite case over time gets flimsier, to the point where it is loose and not as sturdy as when it comes right out of the box. I already can't really use my white case anymore because somehow the screw holes got stripped and every time I try to fasten the case together, it doesn't work right. My G6 lite has been in the black case and GBA case ever since. This brings up another thought too: I have used the GBA case probably a LOT more than anyone else has, because I like to switch between GBMicro and DS Lite and Gamecube gameboy player. After a while of always changing the case, I started just leaving it in the GBA case because it was easier. The GBA case is also MUCH sturdier than the DSLite cases they give you. Anyways, I'm getting off topic, but if you get a G6 Lite, just do your best to take absolute care of it and you SHOULD have no problems. If you do, God forbid, then you should be able to get a replacement for no charge.
Now, to what I like about the G6. I'm obsessed with the G6 icon interface. it's as simple as that. I love it, and if you didn't know, you can create your own custom icons because someone released a very nifty wonderful program. I soon will release a pack of every icon available for both NDS and GBA, even with that hotpink background problem fixed, because like I said, I'm obsessed with the icons and this is some way I can give back to the community.
Also, everyone seems to fail to mention that the G6 touchpod menu has NO INTRO as compared to the M3 DS menu. The GBA menu has the intro, but not the touchpod interface.
I personally love being limited to the space I have on my G6. I know it sounds strange, but I get flashcard diarrhea with my M3. With a gigabyte of miniSD space, I can fit every GBA game I'd ever want to play, or at least twice as many DS games as the G6, and with so much added choices, and with no way to personalize them (the icons...drool), they seem faceless on the menu and sometimes I don't even want to sift through all the games I could play just to choose one, because there are so many freakin choices. In my opinion, the G6 is nice because it FORCES you to think hard about what games you really want to play. Like, for a while, even though I already beat New Super Mario Bros., I still put it on my G6 just on the off chance that I wanted to play it again for like 5 minutes. Well, after a while I got sick of not having Phoenix wright on the card (which I haven't gotten close to beating) so I deleted New Super Mario and Bust a move to make space for it. This was good for me, because it made me choose to have phoenix wright on there as opposed to two or three other games, which will in turn help me to want to play phoenix because now I made it that much more important than..GASP...new super mario bros., one of the best games to be released in a long time. This is just my opinion though, everyone will feel different about it.
With regards to speed, M3 users have to make a big decision about what memory cards to get for it based on their speed. G6 users have the fastest possible memory so thats a tough decision made irrelevant.
One other thing that's good about the G6 is the entirety of the GBA experience. M3 plays the games just as fine, but after experiencing the crazy faster realtime saving/loading of the G6, the M3's is really hard to deal with. Also, I've noticed that with compressing the GBA roms when you put them on the G6, it compresses them enough that the space the rom and its dual sets of three saves/RTsaves is less than just the rom itself on the M3. Add to that the added megabyte of required save space just for one realtime save and regular save for a GBA game, and an extra half megabyte for each realtime save after that and you have much more space needed for M3 GBA games. Games like riviera, a 32MB Rom, will be compressed to 15-16 MB on the G6, with 1.5 megabytes of RTsaves and 768 KB of regular saves and a 8KB icon, you have less than 20 Megabytes of space to put riviera on the G6. Compare that to the M3's 32MB for riviera, 1MB for one regular save/realtime save (with 1MB for any added regular saves), and a total of 1MB for two additional RTsaves, you have 34MB for a Riviera rom, 1 regular save (as opposed to three), and 3 realtime saves. Ok anyway, you get the picture.
Don't forget that with the M3 Lite, you're paying $80-$88+probably $10-$20 shipping on just the unit, then you need to buy a gigabyte microSD card that will AT LEAST cost you $30-$50 just to make the added memory availability a plus over the G6 (if you get 512mb sd card, its like an underpowered G6 for more money...ignoring the corruption problems). Then if you don't have one already, you'll need a memory card reader that you should probably spend about $10-$20, leaning towards 20 so you get USB2.0 and reliability (i use sandisk's microSd/miniSD/SD card reader...$20). Assuming you can get the lowest price in the ranges I suggested, that's still $80 for M3Lite)+$10 shipping+$30 memory card+$10 cardreader = $130 at the very least. If I were you, I'd get the $80+shipping G6 (even with possible corruption) or wait for the possibly amazing DSXtreme and get that instead for the same/less amount of money. (SOMEONE correct me if my price ranges are wrong)
What I do prefer about the M3 is the lack of reported corruption or other problem issues. It's just nonexistant on the M3, which gives you some peace of mind. Along the same lines, if you get a reliable memory card reader, you can easily plug/unplug the memory card into your computer, haphazardly throw some crap on it, and then unplug it before it's done copying and you'll probably still be OK, at least after a possibly needed formatting. On the G6, this would be tantamount to shooting the G6 in the chest and then hoping that the bullet didn't hit its heart. This is definitely a huge upside to the M3 over the G6.
I'll end my crazy long rant with a little nutty analogy: To me at least, the G6 is like a corvette and the M3 is like an overpriced camaro. They're both great to have (maybe a little on the expensive side) but they're still wonderful to use. If all you have is the corvette, you still need a way to drive to work, so you'll drive that to work everyday even with the added risk of a crash and insurance costs, and you'd love every minute of it because it looks pretty and drives faster. If I had a corvette AND a (overpriced) camaro though, the camaro is more reliable and costs less insurance and less to fix so I'd drive the camaro to work everyday and every once in a while take the corvette on a joy ride, wishing that I could drive that corvette everyday with the peace of mind that the camaro would give you.
The same goes for the G6 and M3 I think. If I had an M3 lite, I'd use it all the time just because I wouldn't want to break my pretty little G6.