Ok, so here's my Rytmik masterpiece, I think it only took like a half hour or so to compose from available loops. I've used Garage Band in the past and I think that it is more geared towards the serious musician who wants full time signature control, musical staff notation, accuracy and availability of real instruments and so forth. Rytmik by comparison is firmly rooted in the looping software use model, you build clips out of instruments, where each clip roughly corresponds to a musical measure. Then you drop in the clips in the song editor like lego. Its great for something like a game jam where you don't have a lot of time. Also, the instruments available are (in my opinion) skewed towards game music dev - lots of 8-bit, dubstep, electronic sounds.