Hello,
Here's my take for what it's worth. :)
1. I have never had a problem scoring or editing/creating midi in Sonar. Between the piano roll view and staff view, I have never understood why so many have been begging the Bakers to develop the scoring aspect of Sonar. I've done Symphony stuff, Opera scores....between step sequencer and the tools we have to edit and control midi, I really can't see where you'd have a problem. I've not used Cubase in years, but have always liked the way they handled notation. That said, never a problem in Sonar for me personally.
2. The stability of Sonar depends on how tight your pc is built and maintained as well as how you use your system. A person that has a clue on how to maintain a Windows system that also knows how to set up a Windows system for audio, should not see any show stoppers unless a third party plug is involved that may be problematic to Sonar exclusively. I have been using Sonar since 1997 I believe and though we had a few tough releases coming up through the trenches, to this day it remains my #1 go to DAW. It's extremely stable with a good pc. Heck, it's extremely stable for me when I use my stock Dell Inspiron's using a Realtek soundcards and ASIO4ALL drivers. But some Windows system knowledge is important to get the best out of it. Nothing too intense, but it can really help as well as knowing how to tweak Sonar a bit for your particular needs.
The new Sonar X3 was up running with a project for me for days without a save just to see if I could make it crash as well as how long it would take to crash it. The project was a test project that didn't matter to me, but it never crashed and I didn't save one time for days. This has been one of the best versions of Sonar I have ever used.
3. Incredibly happy. Sonar has more tools, weapons, plugs and instruments that sound great right out of the box than any other DAW software I have. The stock plugs in Sonar obliterate the competition hands down. Granted, some of the other DAW's have some good stuff here and there, but our Sonitus stuff alone crushes just about anything in the others. Again, I've not tried the new Cubase, but I'm basing my opinion from PT 11, Logic, Reaper, Nuendo, Sequoia, and Studio One. All those programs are really good, but I think Sonar is light years ahead of them for how *I* record and work over here.
4. Don't sweat Gibson. The same staff that is creating Sonar will be the same staff that does it until otherwise stated. If anything, since Tascam is a trusted name in the recording industry, it will help to elevate Sonar as well. The Gibson name can only bring more attention to Sonar whether it be good or bad. In this business, it's when they stop talking about you that you worry. :)
5. As of now, no MAC version and it probably won't happen. However, we've been rock solid using Boot Camp on this end without a hitch. So don't sweat it. I really think the MAC vs. Windows thing depends on what your needs are as well as how you use a computer. I've been a Windows man since 1995. I have every system I have ever used (except for one that I sold and one that a storm took out) still up and running perfectly. 2 boxes running Win 95, one running 98 SE, 2 running XP, one Running Vista, 5 running Win 7. All are absolutely perfect, can run the audio programs that were created for their time (some can run later software) and only one machine has been reformatted 2 times out of the bunch.
I'm no computer guru, but I keep my pc's tight and well maintained. I have also done much research on what should be running on a Windows box and what can slow it down. That's one of the things to think about when you compare a mac to a Windows box. Once you disable all the stuff that doesn't need to be running on a Win machine, it totally changes your experience for the better. Every one of my pc's has tweaks to make it run better without over-clocking. I do the normal scan-disc, defrag, and clean out my registry when I uninstall something. I remove folders that are left behind after an install. It takes some work, but once you get used to it, these baby's will hang right with any mac. But again...it depends on what you're trying to do.
We have decent video software for Win machines...but in my opinion, you're better off running a mac for that stuff. But for audio...no machine is better than another really. I like Win better myself though we have two macs. It just becomes a personal preference once you get the Win machine where you want it. Good luck in whatever you decide.
-Danny