GDD5 is better. You sure it is not a GT 650? Because the only difference between 640 and 650 is that 640 uses DDR3 while 650 uses GDDR5. Anyway GT 640 with GDDR5 is like GT650 which is far superior to the DDR3 cards.
+1 this. if you found the same series but different memory, always go for the higher one.