The verdict
For a small B2B software team with at least one engineer, I'd pick PostHog. One source of truth with less glue between vendors is worth a lot, each product has a free allowance, and SQL is there when the built-in reports run out. If the person doing the analysis is a product manager who wants self-serve funnels and retention and has no wish to learn a wider platform, pick Mixpanel. It is quicker to pick up. If you only need website traffic reports, neither is the answer and Google Analytics is enough. Run the same funnel in both before you commit.