Let me start by saying I really just browsed the earlier posts, and didn't read everything so carefully.
I've used ProTools professionally and have Sonar 4 PE at home (with a MOTU 828mkII)
It sort of depends on what you're looking to do and what you're already familiar with. Traditionally ProTools has a bad rap when it comes to MIDI, while Cakewalk was known specifically for MIDI. I haven't used ProTools 7, but I still think it lags behind many of the other DAWs in terms of MIDI capabitlities.
If you go down the Digidesign road, you're married to them forever. ProTools will only work with the Digidesign software. If you get Sonar (or Cuebase or digital performer) you can upgrade your interface to something completely different in the future.
Take some time (you've probably already done this) to compare the hardware specs side by side and see which matches what you're looking for better, and of course price probably plays a role.
While ProTools is still the "industry" standard, plenty of professional music and movie studios are using Digital Performer (also GREAT for MIDI), Sonar and, Nuendo. The field has opened up a little bit. I specifically did not go down the ProTools road for my home studio because of the price and the digidesign propriatariness (I think I just made up a word).
Digidesign use to have a ProTools free, but it's not supported by OS10 or WindowsXP. You can download a demo of Sonar however. To be honest I don't know how the features of Sonar Studio Edition compare to ProTools LE. I've really only used HD systems of Pro Tools.
Oh yeah, with Sonar you can also set the controls to mimic ProTools (or other DAWs), so you can use ProTools commands to control Sonar, which is pretty cool.
If you have a chance, try to get some time with each DAW and see what feels right. ProTools is great, but I don't believe it's a "must have" in your signal chain.