The memory interface will still be 256 bit as this does not double or increase with SLI. Two GTX 760s will be better then one GTX 780 however the one 780 allows you to add another later while the two 760s will lock you in to just having those two cards. It will depend on your long range plans vs. what you can have now.
Never start out SLI if you can afford a great single card. Some current games have had issues with SLI - a lot of current games are console first - SLI is an after thought.
A single card is less power hungry an is an easier job to keep cool. I vote single 780.