My verdict: the veteran of product analytics.
Mixpanel is solid and mature, but PostHog now offers an alternative with more features for a better price. It's a solid classic for product analytics.
But today, I prefer PostHog. Mixpanel's price can become prohibitive for complex tracking, where PostHog is often more advantageous. It remains a very good tool, but watch out for pricing.
What I like less: the pricing that can explode on high-traffic products. And the fact that PostHog now offers analytics + session replay + feature flags + A/B testing in a single tool, where Mixpanel only does analytics.
My advice: if you're starting a new project, look at PostHog first. If you're already on Mixpanel and it works, no need to migrate. But if pricing becomes a problem, PostHog is an excellent alternative.