I think, unless you have some pretty solid empty-hand skills, a "tactical pen" is just something you'll be gripping while you lay there and bleed. That being said, the Parker Jotter in stainless steel is thicker and tougher than the Zebra and it just looks like a pen.
Amazon.com : Parker Ballpoint Pen, Refillable, Medium Point (PAR1333211) : Ballpoint Stick Pens : Office Products
I can't imagine a whole lot of scenarios where someone is going to deploy a "defensive pen" with a lot of success. With that said, it's better than nothing I guess. I don't think anyone is ever going to say "man, I wish I didn't have that darn pen in my pocket."