I would say, go with single card solution as long as it fits your needs. Its always better than dual cards, need less power and generate less heat. But most importantly, not all games or applications are equally good with dual cards and some even may not support. For actual mainstream title, this is no problem.
Best value for the performance would be sli 970s, but they are not future proof as a single 980 Ti I would say. You can combine another 980 Ti later and still have 6gb instead of (even crippled version of) 4gb. If you are still looking for a system, then I suggest to get a power supply with enough wattage delivery for dual overclocked 980 Ti, just in case that you will add another and even overclock them. Don't buy cheap psu...