Having owned O2 Mark and Vixskin Goodfella... I voted for Mustang...
As others have mentioned, the O2 is alot firmer, and the inner core is pretty stiff. While O2 Mark is a well made toy, I don't like it for those reasons.
I much prefer the softer outer Vixskin layer of Goodfella and the more flexible inner core.
As far as the head of Goodfella (and Mustang)... it's prominent but so soft that it isn't uncomfortable at all. Actually, the ridge on the Tantus VIP Supersoft, which looks alot less prominent, was actually annoying for me... and the that's the Supersoft (ie. entirely made of the outer layer O2 material).