Preferred seats are regular seats, just typically closer to the front of the plane. If you already have seat assignments together, there is ZERO reason to pay for preferred. Often, because AA designates so many seats as preferred (sometimes nearly all aisle seats), a non-elite can be "forced" into buying it.

MCE for $90 for such a long trip isn't that bad of a price (often it's around that for just JFK-LAX). But nobody can really tell you whether it's worth it to you. You'll get a couple extra inches of legroom and a free (alcoholic) drink.

If you're going to do it, upgrade your seat now while there are good seats available together. JFK-LAX probably won't book up so quickly as there are a lot of MCE seats, but on other aircraft it's a smaller section and many elites can book it for free.