Hello Tarang,
One2One Integration is a client-side based intergration. That means that the CRM or more precise the browsercommunicates with the Groupware client (Outlook and Lotus Notes) via Javascript instead of the Groupware server which is MS Exchange and Lotus Dominio Server.
Refer similar link
http://scn.sap.com/thread/3222566
http://help.sap.com/saphelp_crm60_bu/helpdata/en/7e/b10af53a064f08a28f60f7581e8c55/content.htm
Server side integration is too complex and not easy to maintain
Client side integration is easy and require less steps to be performed. If the number of users are less, better to use the client side groupware integration which is hassel free and easy to maintain. It is just one time activity on the user desktop/laptop.
I hope this was helpful.
Regards,
Reshma Rao