Neither. Both are overdone and do not stylistically fit 90% of the homes they are installed in. Unless this is a loft in a down town converted warehouse condo? And not gray. Gray is dropping like a rock in popularity.

Waterfall counters are VERY modern and only look good in specific spaces. I would NOT use quartz as a backsplash anywhere near a stove as it can burn fairly easily. It would be fine for countertops, but that would be a lot of gray.

Kitchens aren’t popularity contests and colors don’t go out of style. People that say they do just don’t have the imagination or experience to use them.

I would need to see your kitchen and know a lot more about your style but IMO quartz is my last choice for counters and back splash. A waterfall counter IMO only works is a very contemporary space and no quartz behind cooking appliances for sure. I love polished granite so much more than fake stone and no need to worry about it behind a cooking appliance,

I would nix the waterfall - as mentioned above they are only for very contemporary kitchens. But another issue is that they need a highly skilled installer, which equals expensive and also can equal hard-to-find. There are many posts on this forum about botched waterfall counters. It's not a job for the average counter installer, who would do a perfectly good job on your counter templating and installation, but wouildn't on waterfalls because they are tricky.

I also would not do that counter as a backsplash. First, because it is too much of a good thing. That is a dark dramatic counter, and add that to your gray cabinets, that will be one gloomy kitchen. Second because if you are thinking of it as a backsplash behind the cooktop, that is a bad idea because Quartz scorches when exposed to heat. There are many posts on that topic on this forum too. So, just do that Quartz as counters, and find a light bright backsplash to balance out all the gray you will have.