You should go for Platinum! With the $151 fare that you posted today I'm sure you'll by flying on some other great deals soon.
By the way SFO-PVD (Providence R.I.) is prcing out at $198 + tax at the moment (matching SouthWest). If you take one of those you will nedd 2576 (to JFK) +500 (JFK-PVD) x 2 base miles which would get you Plat on the PVD-JFK return leg and a full 100% plat bonus on the JFK-SFO return leg.