First of all, never borrow a pen from the FA. I saw this listed as one of FA's biggest pet peeves.

You'd be surprised to find that the question of what pen to bring on a pen is actually discussed often on pen lovers forums. One issue with fountain pens is that the change in cabin pressure can cause them to leak in the air. Keeping the pen with the point up during takeoff can help prevent this.

My favorite pen for travel (and for a purse pen) is the Pilot Petit1, a $3 mini fountain pen that takes cartridges. It writes perfectly every time, is super lightweight and has never leaked on me.