As others have mentioned, only UA and AA offer a dramatically different product than anybody else. Unfortunately, this product is only available for flights to SFO and LAX out of New York. Do you really want to connect twice? Otherwise, there are only two carriers that serve BOS-SEA non-stop, AS and JetBlue. AS is fine, as is JetBlue, though JetBlue obviously does not offer first class. That being said, JetBlue is certainly more pleasant than an intra-EU premium cabin, with live TV, good legroom, etc. Just bring a bag lunch. Personally, I don't think there are any carriers that offer anything to make it worth switching from a non-stop on this route.

If you decide to stop, then AA via ORD or NW via DTW or MSP would make a lot of sense, as it will split up the trip nicely, remain on a relatively direct route, and should be all mainline aircraft. CO via EWR wouldn't be bad either, though it will mean a RJ out of BOS and the omnipresent risk of delays in EWR.