307 have a bad reputation for electrical faults......
The 307 had a score of 65 I think, compared to the Megane's 118. The lower the score the better and warranty direct pay out 50% of their bills for electrical faults on the Megane, where as on the 307 the highest percentage (about 20%) is for suspension faults. So according to that, the 307 is a MUCH safer bet.
My Mazda is rated at 48, so is better than both, but I've had more faults on mine than my Renaults.