They aern't really competing.
Kmail and friends are the current stable PIM apps for KDE. Kolab client is a hacked Kmail that was created parallel with Kmail for Kolab server users. Kontact is the future of KDE PIM apps. The PIM team is turning Kmail, KOrganizer, and KAddressBook into KParts. (The KDE component model) These parts will be united under the Kontact UI.
In other words, choose whichever one will meet your needs today, and be prepared to upgrade if necessary to Kontact in the future.