If you want both, buy an adaptor. I got an adapt ore that attaches to the hole in the stand with a plate above that supports 75 and 100mm VESA mounts. I have my 21.5" 4K I have exclusively for my studio on a fully articulating arm that allows me many position options. Like pulling far off the wall and turning it 45 degrees so it's like it's on a non existent desk.
Perfect since a zero g chair (the ones that recline all the way until your upside down) with large casters I put on it to roll around and a theater couch are the only things touching the floor in my hybrid theater/man cave/ recording studio (except when I bring in a drummer to record). My guitars hang on the wall across from the Mac with my accessories in pelican cases slid in a shelf I built that doubles as my projector stand in a 3'x3' closet I added onto the back of the room. Complete with a electromagnetic locked door that is impossible to find when closed.
With the Mac mounted to the wall adjacent to a wall that has one of my 85" Sony 4K's recess mounted so it can give way to my theater screen that comes down when it's movie time. on a screen that comes down over my tv.
All of that was simple. My 7.1 surround system was a little tougher. Ha. Just say my walls are virtually speaker boxes. It's great seeing people jump when it's pumping and they are near what they think is an AC return metal grill is actually a 15" sub. Ha. And that's not getting into the ceiling speakers wired to the tv for casual tv viewing.
Any of you need help designing anything A/V? I just finished building a pair of speakers with 8 4" full range neodymium full range and 2 12" eminence Pro Kappa 12"s in each. I bet no one can guess how I wired them so the 8 8ohm speakers ended up with an overall rating of 6.5ohms... That's above 4ohms and below 16ohms which are the ONLY two possibilities according to 'experts'.
I apologize for my verbosity. The part of my brain that allows me to make a living writing music is also leads to this. A stream of consciousness description of my space when I just set out to tell you guys that for $12, you can make any Mac wall, arm, pole, or stand mountable.
Of course there's another solution I came across when I decided to take multiple Mac Minis and ext HDD's and build a whole home media server. Get the top of the line model and mount it either to the back of your TV(which is the monitor) or to the wall behind your wall mounted TV. I'm a mini fan! Just wish they were record quality. And while they are technically 4K, their fps isn't up to snuff. If you already have a rarely used flat screen like 90% of us or just want to hook it to your main tv, the top model mini is enough Mac for anyone not wanting audiophile quality digi recordings that are near analog quality or don't perform Video editing for a living.