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.
Why? Because you can invest the money and use the investment to donate more later? But donating more now increases the recipients' functionality so that they're able to contribute more to their respective societies in the than they would otherwise be able to in the time between now and later.
If you can donate it to SCI later, the value of that donation gives a lower bound. You also might be able to donate to something else you find.
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.