Well, I do own both and have been shooting with the 1DX for about 2 weeks now. The 1DX that I have is very fast when it comes to AF, but as with all cameras, it depends on the light, lens, AF mode and skill/technique. Assuming you have fast lenses (2.8 and faster) and know how/when to use the AF modes etc the 1DX will not disappoint. When compared with the 5D3 I would say the AF on the 1DX is faster and can be very accurate when used well (and can be very impressive when used in conjunction with its high speed 12fps). As for IQ they are really very similar. You will get a different noise pattern between the 2 (I actually prefer the noise pattern on the 5D3 but this is completely subjective). The 1DX does have a slightly better high ISO performance showing less noise. All in all the 1DX has been a pleasure to shoot with and has more options for customization (such as spot metering linked to AF point selected, 2 M.fn buttons, AE & FE adjustment etc). As I am not a wedding photographer I cannot say if you will appreciate these differences but I don't think the AF alone would be worth the upgrade as in my testing it is best taken advantage of in quick movement (sports, bif etc) but maybe it would be for you. Rent one and take it along to verify.