I think they have similar performance. I am a slight Mobil 1 fan because I often cheered for McLaren, and the past 15 years or so they have had some small Mobil 1 sponsorship/partnership.
Furthermore, whenever I send rebates and/or the Mobil 1 Rewards into Mobil, they get the rewards posted fast and rebates out fast. To me, that is a sign of a pretty good/kind company.
The Mobil 1 guys shouldn't necessarily be grouped in with the entire corporation.
To me, marketing wise, Mobil 1 is psychologically positioned higher than Pennzoil Platinum. I know Mobil 1 has EP, so really EP = PU and regular M1 = PP. Nevertheless, I feel in store placement and whatnot even regular Mobil 1 is positioned as one of the top options. The Mobil 1 name is worth a lot at this point.
I guess some could argue Mobil Super Synthetic = Pennzoil Platinum, but that wouldn't be fair to the superb things we've seen out of PP.