These moves aren't interchangeable. You can only use the sword slice to take out enemies that attack alone, while the flurry of stabs can only take out these smaller little bastards who always attack in groups. Of course, each type of threat lets out it's one sound cue, and there is a specific rhythm to the process of taking them out. 

The importance of that rhythm may seem minimal at first, but that quickly changes, as it's only a few bars into this level that the samurai gets sidetracked. The dude starts thinking about his family, something I guess someone in the middle of a life-or-death struggle against unholy evil may be prone to do. These thoughts manifest on screen as images of his children in happier times. These images obscure the screen, limiting your ability see the demons before they attack. At this point, you must rely on the audio cues that each enemy lets off as the appear on screen, and your ability to stay on beat. 

Other than the addition of the flurry attack, none of this is all that different than Rhythm Heaven (officially titled Rhythm Tengoku) on the GBA, but there is absolutely nothing wrong with that. Out of all the games I played at E3 thus far, Rhythm Heaven is the only one to make me laugh, wince in pain, and bob my head to the beat all in equal measure. Though details are currently scarce on the title (Nintendo couldn't tell me a release date, price, information on additional modes, or any other information on that game), I'm still confident that this one will be worth picking up for diehard fans of the GBA entry in the series.