Currently use X3, and trying to find out what new capability upgrading to Platinum would provide. Is there a summary anywhere on this subject?
 
I tried to find access to a users manual for the Platinum version but was not able to, seems like all the online Docs stop at X3.

yes there is!
 
here:
http://www.cakewalk.com/Products/SONAR/Whats-New#start
 
and here:
http://www.cakewalk.com/Products/SONAR/Rolling-Updates#start
 
hope that helps... 

You could start with this
https://www.cakewalk.com/Products/SONAR/Whats-New#start
 
Then look thru this to get an idea of what a year can bring
https://www.cakewalk.com/Products/SONAR/Rolling-Updates#start
 
I upgraded from X3e last January.  IMO, the performance enhancements alone are worth the price of admission.  Throw in the rest and it's almost a no-brainer.
 
If you're on Producer, check here to see if this deal is still available
http://forum.cakewalk.com/The-deal-is-back-on-16Dec2015-at-audiodeluxecom-m3328339.aspx
 
 

If it's not, it's the same price - today is the last day I think - at JRR shop. You'll need to log in to see the price... 
https://blowout.jrrshop.c...from-sonar-x3-producer

Sorry  - there is an online manual,  just overlooked it.
 
Does anyone know if Platinum includes an mp3 encoder or do we still have to buy that separate?

You have 3 options for MP3 encoding
 
1.  Purchase from Cakewalk ($10?)
2.  Use one included with a previous Cakewalk product such as PA9
3.  Configure LAME (free)