Think of it as a road with 4 lanes and 8 lanes. With 4 lanes there would be less traffic at a given point of time while it would be doubled with 8 lanes. The more the better.
With more and faster memory bit-interface of high value is good one.
Nvidia has recently revised the specs of GTX 970 as many users have complained that beyond 3.5 GB of VRAM, the performance suffers and card can't use its full 4GB VRAM. Now it is mentioned that this card has 56 ROPs, 224GB/s memory bandwidth with cut-down version of GM 204 architecture plus 3.5 GDDR5 + 0.5 DDR3 VRAM.