If you'll be flying AS quite a bit, it makes sense to earn status on AS. You'll get free space-available upgrades (i.e. AS does not charge "stickers" as AA does), use of the FC checkin line, access to the better coach seats, early boarding, etc.

There are several ways to earn AS status. You can fly 20K miles strictly on AS, or 25K miles on a combination of AS and partners, or you can fly 30 segments.

When I lived on the west coast and had AS status, I earned it via 30 segments. I posted all short (under 500 mile) segments (either AS or American) onto AS. This gave me AS status after flying just 15K miles (30 segments). I posted longer (over 500 miles) segments onto AA, because back then you could only qualify for AA EXP by miles or points.