Currently a slot 2 cartridge is the only method for playing gba games.
Here are two in depth reviews of both the Evolution and R4DS,
R4DS Review:
GBAtemp Forum -> R4 DS Review
Evolution Review:
GBAtemp Forum -> CycloDS Evolution Review
Aside from the Evolution being a new product, I don't really see much of a difference.
One note I would like to point out, since the time of the R4 review, download play has been fixed, along with a few other things (through updated firmware).
Also, Gameyeeeah.com has the R4DS for $35 +$5 shipping. I just ordered one yesterday, I bought a DS this past weekend. The only bad part is delivery time is 12-18 days, unless you want to pay $20 for UPS.