The 400 goes to the beaches and fleshpots (Caribbean, Orlando, LA, Vegas etc.) while the Airbus does the Far East and Africa. Both fleets do some East Coast work. Many destinations are swapped seasonally between the 340 and the 744 to 'optimise' capacity. As a generalisation, you'll fly shorter trips more often on the 744, and have longer stopovers and fewer trips (and very few landings!) on the 'Bus.
There are more aircraft in the 340 fleet than the 744 fleet, with yet more arriving, so most slots will be on the Airbus. You may be given the choice - I was when I joined.