R4DS won't support Pokemon Battle Revolution Wii connect play. R4DS team claimed 100% download compatibility, but apparently that's not true. Maybe it's better with the latest firmware, but demonstrably not 100%.
CycloDS does support PBR connect play, and the download play works to boot. Also, CycloDS will support the newer SDHC 4GB+ ram cards, R4DS never will (it would take a hardware upgrade)
The R4DS is a bit cheaper, and it will play most any game (save download play) without incident. If you don't care about PBR/download play, don't care about the 2GB limit, and want to save some money buy a R4DS. As of today, they are in stock most places.
If you want the a card that performs closer to perfect, and don't mind waiting for them to come back into stock, buy a CycloDS.
Actually, I would prefer a G6DS 2GB card over the R4DS. The menu is much slicker, the compatibility is better, you don't have to worry about losing/damaging the miniSD card (serious concerns if little kids are playing) These are presently in stock with the usual suspects.