Hey,
You're on a Mac? Your options:
Cubase/Nuendo
Digital Performer
Logic
Metro
Pro Tools
My personal preference is Cubase because I've been using it for so long, but I'm sure the others are just as good. Programs like Logic and Digital Performer will have a very similar feature set, all three got their start in MIDI sequencing a long time ago and have moved forward with similar strides.
I've heard from people who use it that Pro Tools has TERRIBLE MIDI features, and it is very expensive because you need to run it with their hardware. They just moved up to version 6, maybe they've adressed some of the MIDI issues. Pro Tools is mainly an audio multitrack program, it has a lot of nice features like being able to automate EVERYTHING and its reputation as being the choice editor in big studios. This doesn't mean it's the best program out there, it's just been around the longest.
I used Metro many years ago when it was owned by Cakewalk, it's now owned by someone else...it was a very nice program, had a nice interface and a lot of features, they have an SE version for $60 which will probably do a lot of things you'll need.
The big question you should ask is 'what do I need my software to do' and choose the one which suits your needs. If you're doing only audio editing, a program like Deck, Peak or Spark would be good, if you're doing only sequencing, the programs I mentioned above should work.
Take care,
Nick