The with the GTX 970 is not really going to make much of a difference, rarely would you ever get to a situation where you’ would use between 3.5-4.0GB of GPU memory, it’s going to be higher or lower than that.
As for the other option, UE4 does not support SLI or Crossfire, so it would only be able to take advantage of a single graphics card, get the best one in your budget.