I would suggest you post something in the Store forum, if you haven't already. I would have moved this one over to there if you had created a unique thread for your question.
I DID take a look, and I see no information whatsoever on it for official posting - though I did not do an exhaustive search.
I located a PERSONAL post of Craig Anderton's, and here is the Permalink to his post:
http://forum.cakewalk.com/FindPost/3428782 And here is the text from the above post, to save you some time:
"
All Platinum owners get the same updates and features. If you are an active member, the updates will be the same as the lifetime. Only difference is lifetime means you get all future updates to SONAR. Artist and Professional are still under membership and lifetime updates are exclusive to Platinum." Craig Anderton I think that he would have a really good idea of the above, for him to state it so matter of factly.
I hope that helps you in your decision making. For whatever the worth, I would hope you find a way to end up upgrading all the way to Platinum, as well as picking up the Lifetime Updates offer. It is DEFINITELY a couple of costs to add to a given month's budget, but I believe you would be best served in the long term to make the upgrades (to Platinum, if both cannot be obtained in the time remaining). Then again, if you do not see yourself using any of the additional features and functionality of Platinum over Professional, including 3rd-party plugins, then maybe it doesn't make sense.
I hope that no matter how you choose to move forward, that you find yourself with the tools you need to help you express yourself through music, and above all - have fun. :)
Bob Bone