If I was going to the expense to have stamped slate look patio, I would not choose any color gray. Gray is gray is concrete looking.

I'd do smooth concrete. stamped is too iffy for furniture stability.

Especially if this is a DIY. not all installers care about stamping and color additives,

get samples and watch the process so you aren't surprised at the results. (previous Houzz dilemma postings)

Someday, you may want an outdoor area rug to define the seating area.

I'm not a fan of stamped concrete. I'd allocate the money for that system toward pavers or natural stone.

I would do smooth, stamped colored concrete doesn’t age well, in my experience. I agree with dan1888’s comment.

Can you post a photo of the patio area you're working on? Not sure why you're considering a grey color unless that's the color that best compliments with the home/space. I don't like how smooth concrete ages either so going with stamped. Will likely have a large area rug on it anyway.

We just ripped out our pavers and are getting ready to have stamped concrete done in Roman Cut Slate with subtle color. It's not meant to be a feature but a complement to the outdoors area. Similar to the example below.