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.
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.