What to do, sell S3 now while resale is high or wait for S4 price drop?

Through some good trading, timing, and luck, I got a mint Sprint S3 for $120 to use on Ting. Since it is an MVNO and I have to buy phones at unsubsidized prices, what would you guys do? Should I resell this phone immediately while resale is high and wait for the S4, or keep this phone and wait for S4 prices to drop or buy used on Ebay or Craigslist? I guess if I play my cards right, I'll have to spend an additional $280 for an unsubsidized S4 after selling my S3. Is paying that much worth going from S3 ---> S4?