I think just about everyone has covered the primary differences from a pax point of view. Myself, given a choice, I'll take Horizon over Alaska any day of the week. Not because of the degradation of service on AS (which is still better IMHO than other majors), but simply because of the size of the aircraft combined with great service. There's something to be said about only having 37 or 70 pax on a plane compared to 120 or more. (Though, I don't know that they use the DCH-8Q200 on routes where AS also operates). Fewer people, ala cart, with less carryon hassles. Faster on faster off.

Its my choice when ventruing to Montana v. Delta via SLC, despite the fact that the latter gets me more mileage. (Heresey I know!) Nothing beats the rollercoaster ride on that GTF-HLN leg.