Avoid Continental if you want any shot of an upgrade - Platinums are normally sitting in coach on EWR-LAX.
United would be tough too unless you use miles for upgrades, as JFK-LAX won't be EUA'able once they go to that system in a few months.
UA and CO miles are pretty easy to redeem though, and UA has E+, wi-fi and in-seat power on all its JFK-LAX flights in coach.