Sometimes the SEA/EWR connection prices competitively, sometimes it doesn't. It is a very long trip, however, you can try and use your gold upgrade coupons on the lowest fares - not always easy to use, but it can be done at times. You don't get to EWR until quite late. Also @ EWR, you have no lounge options unless you are a UA RCC member, or have Priority Pass.
For trips where I'd have the time, I'd take AS. For trips where I didn't, I'd probably take DL.