Both, but sales is much more important. I'm somebody that can speak for this because I come from a growth background, running growth for multiple venture backed startups, but dove into the sales space.

At the end of the day, no matter what kind of business you're running, you're going to need to know how to sell. This is important. You won't be able to build the next unicorn with only sales knowledge, but it'll get you your initial traction, funding, deals or whatever you're looking for. Sales is an underlooked space and it is super important to have skills in it. Sales is an art that can be mastered just like marketing.

Learn both.