Well, Greg, you know how fond I am of SHS6. It just plain rocks.
That said, I am getting increasingly more comfortable working with X3 Studio, and rather liking it.
You
may qualify for the "any Sonar version" $99 upgrade price. I can't verify that, because I already had Sonar Studio 8 and X1 Essential(basically Home Studio). Try it and advance to checkout. If you don't qualify, it will let you know. You are not committed at that point, but I'm inclined to believe it may fly, based on the specifics in the Producer upgrade brackets.
Just for grins, I tried upgrading from X2 Producer(which I don't have) to X3 Producer. The system put up a flag, and stopped me from doing it. The filters will not prevent you from spending more than you need to, but they will block an unqualified purchase.
I did it. I like it. I have no regrets. Does that help?