You focus on whichever will ensure growth and competitive advantage.
Marketing is not merely Sales. Marketing is the development of the market and creation of customers.
Your aim in marketing is to create demand. You want to eliminate the need to sell, your products should fly off the shelf with your primary job being operations, supply chain efficiency, and optimization to meet demand.
That MIGHT mean focusing on what’s selling more but it likely doesn’t IF the total market for that product is smaller, of customers really want the other product but don’t know it yet, or competition can kill you for that favored product.
Marketing is the work of figuring all that out. Marketing is figuring out the answer to your question, frankly.
Sounds like you need to actually just do more Marketing: asking customers, market research, competitive analysis, etc. Not promotion! Just working with the market to figure out which of these two directions you should go… or rather, how to balance the fact that you need to do some of both.