Hi bkumarm, as MuS mentioned, there isn't any such table. The best you can do is read the release notes for each version. Splunk usually will also do an announcement blog post that highlights the major functionality improvements.
http://docs.splunk.com/Documentation/Splunk/6.2.3/ReleaseNotes/MeetSplunk
http://docs.splunk.com/Documentation/Splunk/6.3.3/ReleaseNotes/MeetSplunk
http://docs.splunk.com/Documentation/Splunk/6.4.0/ReleaseNotes/MeetSplunk
My advice for upgrading is to get to the latest maintenance release at least for 6.2. If some of the features look useful for the 6.3 release, test first in a dev environment if you have one, and then upgrade in prod.
If some 6.4 features look REALLY enticing, you could upgrade to that version, but keep in mind that ".0" versions carry an added risk of bugs, and it might be better to hold off for at least the first maintenance release, which should be expect in something like 3-6 weeks.
Please let me know if this helps!
