This seems to be a very common choice question. Speaks well to both oils quality and value. I give the edge to MC because I've used it a lot in the 5w20 weight, and the price edge.
That said, again, you can't go wrong with either.
I've got an 03 taurus with 30,000 pampered miles. I've always used mobil 1 full synthetic since 1000 miles. Since joining BITOG I've went down (if you will) to PP and finally switched to the Motorcraft. It gets great reviews and UOA's and I've seen no changes or problems plus I'm saving alot of cash.
On second thought, I still change mine every four or five months, If I was gonna try and go a full year I think I would go with a full synthetic.
I used to run MC at 5k but have switched to PP at 10k. Both serve me well - no deposits, noises or unusual wear. I only stopped using the MC because driving 1k/week meant roughly 1 month OCIs. With PP and PureOne filters I'm spending slightly less money and half as much time.