i prefer BF3, because the map is always evolving. You cant just camp in one spot forever, someone will find away of moving you whether its by a well placed long range sniper shot, or a challenger 2 rolling in and blowing a hole in the building or an apache helicopter just leveling the whole building.
The destruction element really immproves gameplay, but aside that the gameplay itself i think is brilliant. Really involving, with debrit flying around with every explosion and its nice having vehicles around to turn the side of a battle very quickly. Everything from quadbikes to jets, helicopter to tanks.