I have both, I like both, I'd say neither are cool. Or both are. That's entirely your thing.
I've voted scars though, cause as many here have already said, they all come with a story. Good or bad, but we always have something to learn, even if it's a lame scar. I'm not really sure about their coolness factor though, a scar can have a cool story but look terrible, a "cool" scar like the ones you get in movies requires an enormous amount of luck (and often if they look good they are from lame accidents, serious accidents get serious scars, and those are difficult to get pretty). Also, what's the definition of "cool"? I have many, many scars, some of them are from having being stabbed. I'm not particularly proud of that, because being considered a badass is only cool for the people who aren't. So they make for interesting stories, maybe, certainly they serve me as reminders. Have some scars with really "cool" stories behind like almost dying but managed not to (falling down when climbing, crash-landing a plane, that sort of thing) but to me they just show I'm terribly unlucky cause I keep being in these dangerous situations, and/or terribly lucky cause I keep surviving them. But someday I won't, so I'm not so sure they are cool. My "coolest" scar comes from when I almost lost my leg when a piece of iron almost ripped the lower muscles off. The scar doesn't look terrible, but doesn't look good either. It gets the ooohs and aaahs if that's what you're interested in (but also some ughs...). I would say the only scars I'm proud of is the ones I got playing rugby (broke my brows so many times, yet still managed to have them look pretty normal).
I have two tattoos and I love them. I have a tribal sign tattoed in the back of my right arm and a huge Japanese tiger tattoed in my left half of my back. The fact that I don't have them in plain view is cool because they never tire me either. They both also come with a story behind, I got them as reminders too, of different things. I'll say something sissy (if you will) and tell you they are the scars I paid for when I didn't get the physical scars for a reminder. Sort of "soul scars" if I keep the sissy mode on. But of course they have their cosmetic value, since I liked the designs (cooperated in the creation of both of them) and I like how they look. And I'd like to get a tattoo that doesn't come with a meaning behind, for a change. Working on the design now.