Bear in mind that no US carrier has premium economy. UA and AA have E+/MCE which are not separate cabins, simply seats with a bit of extra legroom. They are, however, the exact same seat and the soft product is roughly the same. Unless you are tall, there is nothing about either that really matters and both can be purchased, so I would not make decisions based on that.

If you are self-funded and cost conscious, I would recommend grabbing the cheapest flight which makes your needs and gets you from A-B.