Is there any difference on power charger between iPhone 6 vs 6s?
Both use Lightning cable, so we are good there.
Regarding power, the 12W charger is compatible with all iPhones. See product Compatibility section of 12W charger.

iPhone Models  

iPhone  
iPhone 3G  
iPhone 3Gs  
iPhone 4  
iPhone 4s  
iPhone 5  
iPhone 5c  
iPhone 5s  
iPhone 6  
iPhone 6 Plus  
iPhone 6s  
iPhone 6s Plus  


So you should be confident of charging iPhone 6 or 6s with up to 12W charger. The higher the wattage the lesser is the charging time.