on cryptonight:
56´s 1800-2000h/s  
64´s 1900-2100h/s

64´s draw a bit more power(+5%), most important thing is to get cards with good cooling(more then one fan).
ofc u want samsung memory.
You could biosflash your 56 to a 64 if u want.
For a faster Roi i would suggest 56 and for long term max profit the 64s.
For cryptonight heavy coins FE is bestbuy atm