Either one should be fine.
I use Redline D4.....but used Mobil 1 before that and was VERY happy with it.
If you choose Amsoil....check
www.amsoil.com enter your vehicle information in and it will list the correct products in their lineup for your particular vehicle.
I HIGHLY prefer a synthetic ATF.....but, as mentioned, make SURE that it lists on the bottle that it is for the Mercon V application.
I would NOT go extended drain interval with ANY ATF.......the minimum drain interval is at least every 30K miles........21K miles for "severe service".