I tint all of my vehicles. The biggest mistake I ever made was tinting windows on my GMC Typhoon to 5% because I wanted them "really dark" because you can't see anything out of them at night. Especially trying to look out the back window. Granted, I didn't have a backup camera but I still wouldn't do it. IMO when they're that dark it looks tacky too but that's just my opinion. If those are your two options I'd go 15% and not lose a moment's sleep over the decision.