I’m not sure if I should write about a club that I’ve been running and care deeply about, or my summer volunteering. I did some interesting things volunteering, but nothing unique, but I’m not sure I should write about a small school club. It tells about me as a person, but it’s hardly impressive. (it’s a philosophy club, discussions that i moderate)
Impressive or personal? Personal, always personal. AOs have read enough essays to know the difference between trying to make yourself look good vs. a genuine look into your personality.
Choose the topic that you’re the most passionate about, odds are that’s the essay that will give AOs the best glimpse into you as a person. Don’t worry about trying to sound impressive- write an essay about you as a person as it relates to your activities. I hope that helped, good luck 
I think the worst mistake applicants can make is to try to sound impressive or “unique.” Be yourself. Write as if the reader is in the same room with you and you are having a conversation about something that is important to you. It sounds like the personal interest in your philosophy club is your preference, so good luck!
And there are other opportunities to list your activities, as you know.