My feeling about this is.... I'd rather fly a Q400 staffed by Horizon than a CR7 staffed by Skywest for a flight of comparable distance.
My experiences here in RDM (where Skywest already flies for both Delta and United) has been universally sub-par compared to my QX flights. Which is why I always choose to fly RDM-SEA (wherever) rather than RDM-SLC or RDM-DEN.
Here's hoping that Skywest flying for Alaska does a bit better in the service department than Skywest flying for DL/UA.
Dave