Definitely singleplayer. The way most players look at it is that Uke is much easier to defeat than another player, but that's not the point of singleplayer. Singleplayer is about the quality of the replay, not just winning. Yes you can edit the replay as many times as you want, but that still stops 99% of players from making a high quality replay.
By the way, I prefer playing multiplayer over singleplayer.