Yes, 6-3 is fine on a 175-seater, and much better than United regular economy. Furthermore you can to some degree choose whom you sit next to. Smaller is better.
I disagree. The difference in seat pitch is one inch: 32" on WN, 31" on UA regular Economy. If I were 6'3", I think that I'd be pretty uncomfortable in a 32"-pitch seat for the long flight to Hawaii. If the OP can afford UA Economy Plus, then that is the way to go.