wetdentist
X3 Producer is so good I think I am going to stick with it for a while
One point worth considering is that X3 is a "mature" implementation of the X-series concept. In other words, with X3 putting the main "big bits" in place (VST3, ARA, new video engine, and a radical new way to do comping), I would expect future updates to concentrate more on working within the solid framework that X3 has established. In theory, this would mean less of a learning curve because you won't have to learn something significantly different, like the new way of doing comping, but can learn new features "a la carte." It also means Cakewalk has the breathing room to re-evaluate features that were added in the past, but could use a refresh.
We'll see...but as someone who uses Sonar almost everyday, I've always found the updates to be worth the bucks. It's clear Cakewalk's strategy is to include enough new features so that even if you only care about a few of them, the update is still cost-effective.