Never cared much for post count and always loved the fact that Discourse de-emphasises them, and I don’t see what’s so useful about them that they need to be in the user card either. “Last seen” on the other hand is something I would frequently want to check for.
Not saying post count would necessarily become problematic here, and I’m sure some communities would like it, but I don’t wanna see the user card get to crowded. If I had to choose between the two as a default I’d go with “Last seen”.