Technically, Castrol Syntec is not a real synthetic like Mobil 1. A Group IV PAO (polyalphaolefin) Castol Syntec is widely regarded as a Hydroisomerized Group III .
Group III has some advantages and some disadvantages over Group IV PAO. More slight disadvantages though.
Castrol's claim that using Syntec does allow more horsepower production over a longer period seems to be substantiated....but that's not saying much. All the Group IIIs can make that claim. But is a .00124 hp gain really worth it?
I think they are both good oils. Mobil 1 of course is superior but since I maintain my change interval, I really don't think I would get a substantially greater engine life out of it.
Hell, regular Catrol GTX and MobilClean 5000 are fine oils on their own.