Difference between iCloud and MobileMe
The reason they both still exist is because they are still separate services being offered by Apple. Users of MobileMe prior to the announcement of iCloud paid a yearly subscription fee for the service. Rather than refund those customers, Apple will continue to offer the MobileMe services only to those subscribers who have an active subscription. 

MobileMe stopped accepting new customers around June 2011. If I am not mistaken, it will be until June 2012 that those users will end their subscriptions. At that moment, all MobileMe services will stop functioning and I expect Apple to update OS X so that the MobileMe System Preferences panel will be removed. 

This article by Apple contains more information on the transition from MobileMe to iCloud, as well as a comparison table that shows which MobileMe services will continue to exist in iCloud. The short version is that with the exception of iWeb publishing, MobileMe Photo Gallery and iDisk, the rest of the MobileMe features will be found in some way or another in iCloud. Hope this helps.