I am not interested in added content like synths and fx. When I look at the comparison chart I really don't see much difference between the Professional and Platinum versions of Sonar. Is that the case? Are there any functions where the Professional version has a cut down implementation of something that is quite useful eg tempo mapping or similar
 
thanks, 

One thing I can think of on the top of my head is the Prochannel QuadCurve zoom spectrum analysis flyout that is present in Platinum that is not present in Professional.

Another thing is the Theme Editor that is present in Platinum that is not present in Professional

The Addictive Drums 2 that comes with Platinum comes with 3 adpacks rather than the 1 adpack that comes with the version of Addictive Drums 2 that comes with Professional.

With Platinum you get the Full versions of Dimension Pro and Rapture (Not Rapture Pro) but you get Dimension LE and Rapture LE with Professional.

thanks - a further looks show that drum replacer and vocal sync are absent in Professional
but - I have just noticed the lifetime update for Platinum. I think that will swing it for me. I still want to play witht he demo for a bit longer but am very pleased with how Sonar is looking for my needs
 

Yeah , a lot of items I did not list as you said you were more interested in functionality.  Also, the ability to purchase lifetime updates for Sonar Platinum will cease on Jan 1, 2017.  Who knows whether Cakewalk will make it available again or not.

I would say, if there is any way you can pony up the cash to do the lifetime updates, then go for it. Getting the updates forever man...  you can't lose.