I switched to Quaker State Ultimate Durability from Mobil 1 after looking at the PQIA test results for 5W30, and then switched right back after reading about the potential connection between LSPI in DIT engines and the calcium content of the motor oil used. Quaker State Ultimate Durability 5W30 had moderate calcium levels according to the PQIA, while Mobil 1 is low calcium. My car was actually recalled due to LSPI concerns and I passed the leakdown tests with flying colors...no idea if my choice of oil had anything to do with that, of course!
I think that both are great oils for general use and Quaker State Ultimate Durability is a little cheaper at WalMart, but Mobil 1 5W30 had a somewhat higher TBN and might be better for longer drain intervals. I am basing this on 3 year old test results, so formulations may have changes substantially...