Better phone, better SIM (not all SIMs are alike). It could also be the version of Android on the device. The difference in what apps are being run on the device (especially ones running automatically). Also you would have to factor in usage, age of the device, how often they are each charged. There is just no 'simple' answer.
I would suggest you look at the specs of both devices to help you get a better understanding of the phone difference though. Just to give you more info. You can usually find good info on phones (and compare them) through either https
/www.phonearena.com/ or https
/www.gsmarena.com/