If AS were to choose to join an alliance, they should do a lot of research before committing to one. Both OW and ST has their pros and cons so it's really up to AS to see which one works better in their business model.

In either direction though, AS doesn't have many long-haul international destinations (sans Canada and Mexico) so should they join an alliance, they are at best a regional West Coast partner. That may have it's down sides, but there are many positives from an AS standpoint.

It could bring in extra revenue from more connecting passengers abroad, such as passengers from KE and AF/KLM from ST or JAL, CX and BA from OW. It'd be plus for either alliance as it will allow more freedom for major intra-West Coast routes like YVR, SEA, PDX, and LAX which are lacking on both parts of each alliances' North American member, DL and AA. It may even provide an incentive for an alliance customer abroad to choose AS over say, Virgin America, JetBlue or Westjet. In this sense, it might make business sense for AS to choose an alliance instead of sitting somewhere in the middle between OW and ST. Why should a JAL customer abroad take AS when he/she wants to go from LAX to SEA, when he/she can find a cheaper fare online by booking through Virgin America? Why should a KE customer in SEA fly AS to BOS when they can find a cheaper fare on JetBlue?

Another benefit would mean expanded benefits for AS frequent flyers when they fly those international carriers. A person who racks up lots of miles to become MVP Gold might qualify to become an equivalent of a Skyteam Elite or an Oneworld Sapphire, which brings in benefits like extra luggages, priority check-in, boarding, seating, and lounge access when they travel abroad. That meaning, should AS go with ST, MVP Gold members would have reciprocal elite benefits when they across all ST carriers, and same with OW if they choose to join theirs.

Probably the most largest benefit of all would be the expansion of award benefits, more choices and flexibility for the most frequent flyers of AS. An AS member saving those miles for a trip to Cancun, maybe able to grab more expanded SuperSaver awards like taking an Aeromexico or a Mexicana flight instead of just AS.

Plus, both ST and OW have great round the world award mileage products which one can use their own FFP's miles to visit various destinations around the world for a reasonable mileage amount. For example, AA's oneworld award has a really nice structure where it's possible for a person to take a trip around the world, up to 16 segments in all, for a little as 130,000 miles all in business class. Of course, they are subject to availability of partner carriers, but there's a tool on BA's website which allows one to search award availability across all OW carriers, so it's not that difficult, especially when planned ahead far in advance which there tends to be lots of availability. An MVP Gold which earns 100% bonus miles who travels frequently between SEA-JFK can earn that round-the-world business class ticket in as little as 14 roundtrips.