You can use both at the same time. Each activity gets its own set of virtual desktops. I use activities to switch between low-distraction and high-distraction

You can use both at the same time. Each activity gets its own set of virtual desktops. I use activities to switch between low-distraction and high-distraction modes; combined with a 2x2 grid of virtual desktops, I have certain important things on the north-east virtual desktop on all activities, but other things will be only in one or the other activity.