In my experience there's a lot of overlap.

The first matte painters I worked with were all traditionally-trained artists. Illustrators and fine-art painters. Since we could draw/paint we would get asked to do concept art as well.

We worked very closely with compositors and designed a lot of workflows for getting our work into Nuke so the compositors could take it to the next stage.

I don't mean this in a disparaging way, but typically the compositors I worked with didn't usually draw... I did work with one comp artist that was also an amazing illustrator, but usually the compositors who were more interested in painting would generally join the matte painting team.

Although later they did hire more people to the team that would be considered more centered around Nuke compositing or 3D environment work.

We had to learn a lot more Nuke stuff as time went on to bridge the gap between ourselves and the compositors.

I guess it depends what you're most-interested in and what you want to sink your time into?