I shot 14-24 on D800 through a friend, all I can say is WOW.
My vote is 14-24. I wish Canon can match or do better than current Nikon 14-24.
There are already lenses for Canon bodies whose optical quality surpasses that of Nikon's 14-24.
Read the OP. You cannot mount a Canon lens to a Nikon Body!!
You should also tell us which superior 14-24 lens is made for Canon bodies that is better so we can avail of your wisdom.
The Nikon 14-24f/2.8 as far as a WA Zoom is concerned, has no pears, I purchased the D800 sometime back as a possible replacement for my 5DMK II gear for Underwater Photography, I already had the 14-24 and was using it with the Novaflex Adaptor on my Canon Gear, works fine but you loose the autofocus.
I have since sold my D800 and Nikon lenses, the D800 in my view simply didn't live up to the Hype and have since Housed my 5DMK III, But I have held onto the 14-24, until Canon bring out an equivalent this Lens remains the best WA Zoom on the Market (as long as you don't mind manual focus on a Canon Body), Canon have nothing to compare.
And before everyone leaps on my back, keep in mind we are talking about WA Zooms here, not primes I have the 17TSE & 24 TSE + The Zeiss 15f/2.8 Distagon T, all absolutely brilliant lenses for IQ, but they are Primes, the Op is talking Zooms, and the price point of these 3 Primes I've mentioned tend to keep them in the Camera Bags of those with True, nothing can help, addiction.
But, back to your question, get the 14-24 if it's in your price range, there's a big difference in price and IQ between the 14-24 f/2.8 & 16-35f/4 Nikon Lenses, I don't know about the IQ of the 16-35f/4, but I cant see it being equal to the 14-24f/2.8, nothing much is.