If/when B6 adds SEA to Mint (and/or allows through-booking from somewhere other than JFK/BOS), I'll seriously consider them. The main problem at the moment is that Mint is a Transcon-only product from airports that are not convenient to me (I have trouble justifying taking the train up to New York or to connect from SFO/LAX-Seattle, and this is from a train lover). I really wish they had some sort of proper premium offering on something below a TCON, but alas...
As such, until then DL is going to hold my business, notwithstanding an attempt to requalify as Gold on AS...and that's mainly to facilitate burning my large pile of VX miles.