The Samsung Galaxy Mega 6.3 and the Samsung Galaxy Note II may both belong to the 'phablet' category, but cater to two very different audiences. The smaller Galaxy Note II offers precise input option of a stylus, and sports more internal storage, plus it takes better pictures and video, all for a slightly lower price. Meanwhile the larger Galaxy Mega 6.3 is meant mainly for media consumption and for those looking for the largest screen in a slim (somewhat) portable package.
You can read a detailed comparison of the Samsung Galaxy Mega 6.3 and the Samsung Galaxy Note II on PhoneArena.