by 20th september your S1 will be worthless.
if 100GH/s@1W/GHs is worthless on the 20th of sept then the S3 is almost as worthless as well, might just skip buying hardware altogether then!
S1 is 2.22w/Gh if we have a 30% difficulty increase by 20th Sept, then the S1 will be near it's end.
S3 is 0.80w/Gh That leaves the S3 (or S1 upgrade) as capable of earning a return until mid December.