The 780's would absolutely offer better raw performance. That said, you would run into sli requirements then. Such as at least a 1000w well rated power supply, you'll need absolute great case airflow for those cards as well. The 780 and 780 ti are beasty cards, require a lot of power and kick off a lot of heat. While the 780's would offer the best pure performance, they would also be the most expensive. You'll need $500 per card x2, plus a very good psu, $150. Must have great airflow as well so may need to ass a few fans.
where as the 780ti is a just great awesome card, you'll spend $750 on the card, while it wont be as good as the 780 sli, you can use a lower wattage psu so you probably wont need to upgrade, you wont run into airflow problems like you would with 780 sli. No matter what your running, the 780ti should handle it. And then years down the road, you could then go the sli route with the 780ti. Vs having to upgrade your gpu's themselves like you would with the 780's in sli. You could always add a third 780 down the road, but then your talking massive power supply and again huge cooling issues in which liquid cooling would be the best bet.