I think it's more important to change the oil regularly than to worry about which is the absolute best.
For a purely street driven car, it is a myth, possibly pertutated by big oil, that drain intervals need to be every 3000 miles or less under normal driving conditions. Oil analysis proves this. I run my 01 GMC 5.3L pick up, bought new, 20,000 miles between changes. There is 123,000 miles on it now. I use a quart every 10,000 miles or so and do oil analysis. European drain intervals are in the 12-15,000 mile range, and have been for a while. They are governed by a stricter standard--the ACEA. Amsoil meets all these standards. I would guess Mobil 1 does too. For cars driven harder, oil analysis is a good tool.
The oil test website referred to above by Big Carrot is an intersting read. I remember this site from a year or so ago--before they tested any Amsoil. I might note that the Amsoil being tested against the Mobil 1 is just your basic 5W-30 synthetic oil. I use Series 2000 0W-30 in the Viper, which is the best oil they make. It advertises a 35,000 mile drain interval under normal driving conditions. I have been changing mine at 6000 relatively hard miles, with oil analysis. The report comes back saying the oil is perfectly capable of more mileage. I'll guarantee Mercedes and BMW would not have extended drain intervals if there was risk of engine damage....especially under warranty.
Steve