I fly that route (or JFK-LAX) fairly often, and my take it that if you're not going to make the 50K threshold, and you're purchasing reduced fare economy, don't fly this route. Economy on P.S., aside from the in-seat power, is probably one of the worst products around. If you're going to make 1P, then you can expect to fly economy until you reach 1P, and after that, clear the upgrade list about 50% of the time. Plus, you'll only earn so many upgrade certs and that will restrict the number of available upgrades.

If you plan to do some international travel, then consider UA, and throw in another flight somewhere to hit 1P. Otherwise, fly Jet Blue for good coach or Continental to have a legitimate chance at an upgrade (which will be difficult at sub 50k miles).