ninja'd. reaper is more for recording but you can also do digital composition with it. renoise is almost fully geared towards just making music on your computer but also has the non-standard interface. FL is a good middle ground, it actually started out as a drum machine and became more fully featured over time. if you use FL that also helps because I use that program the most so I can answer questions more directly, it also probably has the easiest piano roll out there
FL also has an internal soundfont player so if you use that ignore my advice to get sfz and just get the samples