I enjoy both, but for different reasons.
Playing with friends is usually more relaxed and social. There's always plenty of banter, laughs, and memorable moments, even if the stakes are low. It's a great way to spend an evening.
Playing with strangers, especially online, feels a bit more competitive. You don't have any reads based on personality or past games, so every decision is based on how your opponents are playing. It can also be a good way to improve since you're constantly facing different styles.
If I had to choose, I'd probably pick playing with friends for the overall experience, but I like mixing in games against strangers when I want more of a challenge.
What about everyone else? Which do you prefer, and why?