Personally, I prefer a helmet mirror. Riding against traffic means that you reduce the reaction time for drivers. If you’re going 15 mph and the driver is going 30 mph, you are approaching at 45 mph. If you are both going the same way, the driver is approaching at 15 mph, giving three times more time to react. It also tends to place you in spots on the road where you are not expected. A helmet mirror isn’t as good as a straight-on view, but the tradeoffs are worth it.