it's really not that big a deal to update the stuff. Most the time you only need to update the software, with M3/G6 they uninstall the old one and install the new one at the same time, then you go and convert your games to what everyone says you need to and you're good to go. Even if you need to update the m3/g6 firmware, its not that big a deal either, and really easy to do and doesnt take long.
M3 and G6 also have safe mode which seems to work initially for most games, so if you don't know what to set, you can set safe mode and no trim and it will usually work. Lego star wars is a big exception to all carts (except ezf4??) so that shouldn't factor in to your decision. Edit: I guess with the new megaman ZX not saving, thats a factor too, but I remember M3 team releasing the Mario basket fix within a day or so. For me, this was no problem at all since I check GBATemp and other flashcard websites like 10 times daily so I was aware of the fix nearly as soon as it came out.
Think of it this way: No matter how you slice it, DS-Xtreme will cost you about as much as a new DS Lite. Even if you decide to get DS-Xtreme in the future, you should still be able to use a GBA Slot2 flash card with the DS-Xtreme's passme capability, so why not get a cheaper cart to begin with?
If you go with this way, I'd initially recommend you get a supercard if you just want more space for DS Games, and especially if you are getting a card just to last until you decide to get a DSX or not.
But then I remember that SC is not superb at playing GBA games, so if you do decide to get DSX (i do really think it will live up to the hype), then you might want to consider M3 lite or G6 lite.
I have a g6 lite and I love it to death. Great good looking interface, keeps me from having flashcart diarrhea (putting every possible game on the card just because i have so much space, then not wanting to play a game sometimes just because theres so much to choose from), and I take care of my stuff/have had no corruption problems.
I also have an M3 (minisd, not lite) and what I like about it is that its damned easy to not give a crap about transferring games and such, no eject cart or G6 nonsense, and the only thing I can imagine going wrong is if the spring that holds the miniSD in stops working for some unknown reason. But I like GBA games, and so I always prefer G6 because it lets you compress the roms, i like making my little fancy icons, and realtime saving is loads, and i mean LOADS faster than M3.
Regardless of how much I like M3 and G6, Supercard lite is probably right for you. I got one of those, and gave it to my girlfriend, who doesnt give a crap about icons and whatnot, and mainly plays bust-a-move, tetris, jigsaw puzzle game, new super mario, etc, and would never play a GBA game just because why settle for a one-screen game with 6 buttons if theres a two screen game with stylus input and 8 buttons? Also, remember the thing I said about your possible future DSXtreme pairing well with a supercard, since you said you have a F2A for GBA games.
Sorry for thre long post, but this is a topic that I always love to get into really deeply!