With Even more space, you have a chance of the seat next to you going out empty because those are the last seats to fill. If the flight isn't full, then you will have lucked out. I often buy Even More Space if the regular seats are filling up. I see it as a gamble for the middle seat, and if I lose, I still get amazing legroom. If you buy two seats, you can guarantee that. There are 9 seats (one and a half rows) blocked on the A321 for wheelchairs/medical equipment and for families that are assignable at the gate, so don't worry if there aren't two seats together on the seat map.

If Mint is available, and it costs less than two Economy (Core) seats, you might want to just book a Mint seat instead. (SFO-JFK is a Mint route). They are plenty wide and it's a lie-flat transcon Business Class experience on par with the major US airlines.