Aside from better payload and 6 wheels contact points on the ground, a 6 x 6 is better. However, in real-life situations, it is the driver and the line he or she takes makes a BIG difference. After rock crawling all over the country in the last 20 years, I have seen good driver skills outperformed those who have a better-equipped rig. A 6x6 is nice to have but does not end-all by no means. Every choice comes with compromises.
In some situations, a 6x6 will not perform better any better than a 4x4. The articulation of the rear 4 wheels in a 6x6 is very limited. The rear 4 wheels can easily become a one-wheel drive. If possible, I would take a locker particular ARB which gives you that 100% control on demand.
Having said that. why would anyone buy an expedition vehicle to do hard rock crawling unless the intention is to produce a jaw-dropping video for youtube viewers? There are other rigs that will do a far better job. Expedition vehicles are all bout preparedness for the unexpected emergency. In other words, he or she should have plenty of time to know their shortcoming and understand their calculation risks to avoid those emergencies.
So a 4x4 , 6x6 or 8x8, which is better. No one knows but you.