The funny thing is, photography is precisely how I pay my bills. So, I guess that qualifies me as a professional. You had no way of knowing that, so no offense on my end.
My wife and I run a photography outfit, and our bread and butter is weddings. Most wedding photographers will agree that the vast majority of the shots taken during the course of the day are irreplaceable memories. Contracts protect you financially. Dual card slots protect your reputation and, as mentioned above, our client's memories. I learned my lesson early on... The first unpaid wedding we shot as a favor to a family member, I had a card fail on my 6D. Luckily, I was able to recover the files. However, it's not something I am ever willing to risk again. Two cards writing raw files simultaneously are an absolutely mandatory requirement.
My wife has had a D750 since it was released, while I've been chugging along with my Canon gear hoping that they would release a FF with a tilt screen and two slots (I had the 70D for a spell and loved the flexibility that it allowed). The 6DII will be close, but no cigar. It disappoints me, as there's a lot that I do love about Canon.
As for Nikon and Canon compared, I am quite happy with the lenses available to both. Are there are better options on either side? Sure. As far as my new D750 is concerned, the VF AF works like a dream and really doesn't miss unless I do. The 3D tracking is far superior to Canon's sub par ITR that I've tried and really don't trust on my 7DII. To me having the tilting screen is a huge plus when shooting overhead shots during the reception, and when getting candid shots shooting from the hip. It's kinda hard to frame a shot when you can't see what you're shooting.
The great outdoors and wedding venues are our studio. So, in house studio work is of zero consequence to us. I could not care any less about tethering. I require equipment that gets the shot out in the field, that protects my client's memories (hence, dual slots), and has the features important to me.
Not to mention... It makes zero business sense to pay more than double to get a 5DIV that doesn't even have the features that make my life easier (flip screen, better sensor still for high contrast scenes, & spot metering linked to AF point), is also heavier (a big deal for all day affairs), and has a less comfortable grip (also a big deal for all day affairs). So, I was really really hoping the 6DII would check the boxes.
