FYI, the S1 is discontinued by Bitmain, so you'll have to get it used or second hand.
If you can get a few more S1s, good for you.
The long sighted or wise move would be an S2:
A short good news for the S2 buyer is that we will let the S2 to upgrade to 1.8TH/s or 2TH/s at very low cost which you can only image now in the August or September. It still depends on many factors to let us achieve that and detailed plan needs to be drafted, But we are very confident that we can make it.