There is no right or wrong about whether a taxon is a species or not when they're this closely related. There are a few people who give excubitor full species status, but most consider it to be a subspecies of O. alpina. However, you should make up your own mind after studying long series of O. a. alpina ad O. (a.) excubitor, and preferably studying their habits in the wild.
I've collected a good number of excubitor on the North Slope but I've never collected O. a. alpina, and I don't study this group so I'm not really qualified to comment. If you don't have the opportunity to do this research the best thing to do is decide which of the "experts" you trust and go with them.