In my experience, the three major airlines tend to be about the same for domestic flights, with Delta being a little better for cabins and customer service and American having tighter connections and worse delays. You also might have more options on Southwest if you're willing to drive to BWI or direct options from New York. Flights direct from the east coast to Hawaii tend to use widebody planes, which are more comfortable than the other options and sometimes have better legroom in economy.
I'm 6'1" and have done extra legroom flights on some long routes, like non-stop to Asia. I generally think it's not worth it if you have aisle access. You'll have plenty of time to walk on the plane and during a layover.