Upgradeability for me is a "fake" feature. Generally speaking, when I decide I need to upgrade my processor, the motherboard needs to go with it. This is just due to the number of enhancements/additions that have been made since the last time I upgraded. But then again, I typically upgrade every 1-2 years, whereas others might every 6 months or so.
So then you're really down to price and features. Prices for 939s have been eroding, so that barrier to entry vs. 754 is doing the same. Then you're talking about features, SLI being the major advantage for 939s. What people don't seem to like to talk about though is that for the cost of 2 6600GTs and an SLI motherboard, you can get a "plain" motherboard, a 6800GT (which will in a lot of cases outperform the 6600s in SLI) and maybe even add a game or two, all without needing the game/drivers to support SLI, 2 of the exact same cards, etc.
Sure, if you're the type of person that will get 2 6800 Ultras in SLI with a 4000+ under the hood, then go for it. IMHO, I'd take a long look at overall price/performance and certainly trade off $100 for a 2-5% decrease in performance.