Price and availability are the deciding factors for me.
I refuse to have to wait 3-5 days for 7qt of Redline to be delivered via UPS. Mobil 1 I can get at the local Schmucks store any time I want for a reasonable price.
I don't think I am imagining it- I have owned my stang since new. I run AMSOIL Series 2000 0W-30.
I like having it delivered via UPS. Making a trip to pick up oil is a waste of time imho. I always know when I will need it, so a three day delay is nothing too me.
FYI they also have a 'preferred cust program' where you pay $10 and they give you wholesale pricing.
I used Mobil-1 for years in everything I owned and decided to try Amsoil in my Mustang. Can't say I noticed a difference, but that just means it's working.