We use PLESK for our servers, and are thinking of purchasing a CPanel server for variety.

I see some NOCs offering Cpanel 4.8, but the latest version is CPanel 4.9

What is the difference and how easy to upgrade to 4.9? Also is the upgrade free?

Cpanel has the option of upgrading itself nightly, and since you pay a monthly fee, upgrades are included, so you don't really have to worry about the version.