I'd prefer a side load as I've already had my 2gb Micro SD Delivered and I just know that It would fall out of a front load one day and would be immediately lost. its so small its crazy.
Its personal preference I know.
I'm also paranoid about losing the Micro SD. The SC Lite itself, I wouldn't be worried about losing, since it's not easy to take it out once it's in your DSLite (at least on my friend's, it's stuck in there
).
M3's side loading allows it to look like the GBA cover that comes with the DSLite, flat smooth surface. M3Lite is supposed to come with black and blue replacement covers as well. The SCL has a horizontal line and MicroSD hole. Many want thier cart to be as low profile as possible. I think SCLite just comes in white? Anyway, these cosmestic aspects alone probably aren't enough for most people to spend more.
Other things to consider: M3 has better GBA game compatibility (I know some people say they don't care, but GBA has tons of great titles), better built-in movie player (which from the few videos i've tried on it, seems better than moonshell dpg) and custom skins.
I don't know what i'll get, i'm still using the older M3 and SC products for now. I'm leaning toward SC since I have my old M3 if I want to play GBA, plus movie playback is not a big deal to me since PSP (especially homebrew PMP format players) does it better. Most of my DS saves are for M3, I hope I could convert them somehow.