I'm very confused by this post, and not sure about it because of the username "forumMuppet", but here's me trying to attempt to give some sort of serious answer:
An advantage of the 1Dx is most definitely the FPS and AF speed, but also the dual CF card slot comes into play, too (rather than the CF and SD slots in the 5D III). Unless you're going to print very largely, I would not worry about the 4.2 megapixel difference. With the kit you have, I would not worry too much about performance differences as they would not be significant enough to make you wish you had bought the other (aside from that 10FPS), but if you have the option, I don't see why not. And I don't understand why a gripped 5D mark III would be any better than a 1Dx. The battery grip does not improve performance (and only offers the dual battery slot). If you choose the 5D III, you can afford to get another lens just for the fun of it. If you go with the 1Dx, congratulations, you basically have the king of full frame photography in the Canon world.
If I had a 1Dx and 5D mark III, I'd only ever take the 5D Mark III (gripped) when needed as a second body. If I was lazy like I normally am, I'd just take the 5D III without a grip. Basically 1Dx > 5D mark III