You'd have to check the list of plugins in each of those version.
The only one I know for sure is that if you want to use VVocal it's not included in x3 so you'll need to keep it from your X1 installation.
There are no features to carry over only plugins. The extra content is the same and the version of Dimension pro is now 1.5 so if you want to use it check your current version and update if necessary.
Go to the Cakewalk site and under support look for info regarding anything special required for a Dim Pro update.