It'd be best if you write the session data to database AND to memcached... because logging out users randomly if you use only memcached probably won't make it best user experience :) Of course if you'll have evictions which will probably happen.
Using MC with DB you can speed up reads and users won't be logged out if you restart MC or it'll run out of memory... just check if the data exists in MC then if not try querying the DB. You can also speed up writes this way... if you add soft timeout + changes counter.
