What's the difference between ipad mini 5w power adaptor and ipad mini retina 12w power adaptor?
1) May I use 5w power adaptor on ipad mini retina?
Yes but charging will be much slower. My mini Retina came with the 10w power adapter (NOT the 12w) which is no longer listed on Apple's website as an accessory (only the 5w and 12w are now shown). I believe Apple may likely switch over exclusively to the 12w to be included with the mini Retina maybe when the 10w adapters are depleted as it seems a bit silly to have both a 10w and 12w power adapter.
2) May I use 12w power adaptor on non retina ipad mini?
Yes. Using a higher wattage power adapter is completely safe for lower wattage devices like the iPhone. My iPad mini (non Retina) came with the 5w power adapter but I regularly use the both my 10w & 12w to charge. Charging time will be reduced when charging lower wattage devices.
In my experience, the included adapters for devices are:
iPhone (all): 5w
iPad 2nd gen: 10w
iPad 3rd gen: 10w
iPad 4th gen: 12w
iPad mini: 5w
iPad mini Retina: 10w