I had a similar situation 5 years ago. I went with Southwest, 32 years until retirement. I was the most junior guy at the company at #7500, 4 years later I have 2500 pilots junior to me. We still have 5K to grow to get to AA’s size, so if you take the SWA class you’d need to account for growth as well as retirements. I’m not sure AA is growing past the 15K mark.
If you haven’t already made your mind up, and you want to know my reasons why I chose SWA over AA, feel free to PM me. This thread is pretty slanted and biased in my opinion. But, people can write what they want.