Probably the eWin Expansion pack as AFAIK it only contains RAM, so you'd have to download the ROM each time that you want to play, but it loads FAST.
The DSTT/TTDS Expansion Pack is just a re-badged eWin Expansion pack, so you could get one of those too if you could find it sol separately or get one from someone else for some reason.
Also, someone here said that the M3 DS Real's GBA/Expansion pack is also just a re-badged eWin expansion pack, but I'm not sure about that one and the only place I've seen them selling it separately has the same price as a 3in1 while sky2k.com sells them for c.$14 and dealextreme(or one of those other Chinese places) has them for c. $12.
The 3in1 really isn't that bad either as the VAST majority of games WILL fit into the 16MB of PSRAM, and while flash games to NOR flash DOES take a while, exactly how often will you be swapping out 32MB games anyways? Also another to be aware of is that EZ-Flash is changing the flash chip used in the cart which is going to require software updates to use the cart with 3rd party homebrew loaders. Also you MUST patch GBA ROMs before use with the 3in1, mostly use gbata unless they're 1024kbit flash type saves in which case you'll want to use the EZ-Flash IV vlient patcher. The 3in1 is usually in the $19 - 22 price range.
The eWin has it's own loader which I think is in the file section here or available through their forums. Another bonus of the eWin is that you don't have to save patch your ROMs as the loader does it for you, and AFAIK it now works correctly with all save types with the v1.6 loader. (Earlier ones had a problem with a few save types found on some ROMs.)