It seems very unlikely to me that the expected value of donating to SCI is precisely between 1/2 and 1 times as high as the best alternative.
If you know that you can donate to SCI later, the expected utility of waiting would have to be at least that of donating to it now. Certain things can cause it to decrease a little. If there's a small probability of finding something better, it would increase it slightly beyond this. If it's slight enough, it's less than twice as good.
As of today, GiveWell has two new top charities:
1. Against Malaria Foundation
2. Schistosomiastis Control Initiative
Their previous top charity, Village Reach, no longer has much room for more funding. Which is a good thing: partly through givewell's influence they now have as much money as they can put to good use.