Not Sure about Nikon D800 or Canon 5D Mark III? Rent!
LensRentals.com rents not just lenses, but camera bodies. So with the Nikon D800 costing ~$3000 and the Canon 5D Mark III costing ~$3500, it might make sense to rent one or both before making a buying decision.
Thank you for using the links to B&H Photo from my gear pages when you buy any of your photo or computer gear. See my thank-you offer.