From a domestic standpoint in the US, there is no "better accommodations", the service is roughly the same. You can get a seat with "more room" if by "more room" you mean "2-3 inches more legroom and nothing else." It's nice that I don't have to type on my laptop with the keyboard in my chest and screen on my tray table in Economy Plus (United product), but it's not really "more room". I still don't get an armrest, and it's still impossible to exit the row without making your fellow passengers get up.

True Premium Economy (such as what exists on Lufthansa, Air NZ, and now Delta international routes) does not exist for US domestic travel.