can you tell us the clock speed on both of them or send us a link.
the 4GB of vram is not relevant as there is not enough GPU power to use 4GB (i.e. the frame would be so slow once you've added enough effects to use 4GB that you wouldn't do it).
I'm wondering if there is an interim generational update here, otherwise the 745 is the better bet.
BUT you are gaming on a laptop, be aware that this will limit you.