American Airlines is the only airline that I am aware of that gives you lifetime status based on an accumulation of miles from any source. Because of this, initially I transferred my SPG to American to get the 5K bonuses. American, at the time, had all kinds of bonuses of its own. There are challenges which also give mileage opportunities. Once I achieved lifetime platinum status I started saving my SPG for hotels. However, somewhere along the line BA had a double points promotion which meant that 20K SPG points turned into 50K BA miles.
So I would suggest reading some of the threads of the airlines and see if you like how they work. You, I see, are also from Chicago. This is a good location for AA flights to California, Mexico, east coast. They fly most of where I fly. They also do not charge to change your AA Award ticket.
So, I would say consider AA, and that between SPG and AA you could end up doing very well.