How do you prioritize your research - Buyer vs User?

It really depends on the stage/GTM motion your company is in. You might want to focus on the user if you are really focused on product-led-growth (PLG) and reaching that point of 'activation' - aka the aha moment when a new user gets the value of your product and why they need it. If you have no idea what I'm talking about I'd suggest exploring this article/site from the product-led collective- https://www.productled.org/foundations/product-led-growth-metrics. Other scenarios where you may want to focus on the user is if you're seeing typical product usage metrics declining (DAU, WAU, MAU, time spent, L21+/28 - percent of users are active more than 20 days of the month) 

Prioritzing research with buyers may be more helpful if sales if you're exploring a new sales/GTM motion like moving up market to enterprise, or down market to self serve, targeting new buyers outside of your core buyer, landing/expanding, cross-sell etc. It also can make sense to do research almost exclusively with buyers but also sprinkle in some power users who know your product really well and are a big champion of yours. 

Prioritizing buyer vs user research I think ties into broader business goals. Do you need to power acquisition efforts and broader brand teams? Then I'd prioritize buyer research. On the other hand, if you have an acquisition machine that is generating plenty of sales but retention could be improved, then I think looking at user research needs to be prioritized in this case.

As with almost anything the answer is a bit grey, because there are plenty of examples where both types of research can help. If you work for an established software company, you may have a UX team which does some user research as well which can naturally help prioritize.

If your company does have a UX team doing this research, I'd recommend building a strong relationship with that team and sharing all the research you are doing to help inform all the efforts across Product Marketing and UX.

Think about where the bigger pain is for your organization. In product marketing lack of resources is usually the case, so you need to prioritize based on the needs of the company. you have major issues with churn I'd suggest starting with user personas, on the other hand, if sales are struggling to close deals, the buyer personas should be your priority.