I had the same dillema, for me the only factor was which had the best price/performance ratio at the moment, so obviously I went with the 754.
As for 939, the performance increase is marginal. Whatever you do don't buy for future upgradeability, sure there'll be dual-gpus and dual-cpus but with it comes problems, overheating and shreaking loudness comes to mind. 754 is more than you need now for whatever you're doing, cheaper and more reliable.
All that aside if money isn't an issue, get 2 6800 Ultras and 939 SLI.