MaxMSP is my software of choice for composing as well for sound and video installations.
If you wonder what MaxMSP is all about, take a look here.
Some externals I programmed for MaxMSP:
rs.delos, sort of a crossbreed of the pre-Max5 timeline object with a pianoroll style Midi editor, lets you put events in a timeline and send them out synchronized to a transport object.
Click on images below to see rs.delos in action:
Available in the Store !!!
Download a Demo version: