Metal roof over existing wood shake

currently I have wood shake roof and would like to replace with stone coated metal roof. Here are my questions 1) Is stone coated steel an upgraded from wood shake in both value and quality? 2) most contractors in my area recommend to install metal roof over an existing wood shake roof. Is it technically correct? What would you recommend? 3) What are the concerns of installing metal roof in the cold weather during the winter? For example like in 20 degree Farenheig 4) I do not see any vent in my soffit and assume the wood roof act as a big one. Do I need to install more vents at the soffit after putting the metal roof? Thanks for your answer in advance

+1 to all of Todd's comments (he usually beats me to the punch). 1. Steel will last exponentially longer and will not require a majority of the periodic maintenance woes of shake. I do love wood shakes, but I would never put them on my home. 2. Going over the top of shakes is permitted by just about every manufacturer that I know of but you will need to check with your manufacturer. While I appreciate the approach and am in no way trying to undermine your contractor, I always prefer to strip the roof. Metal is basically a permanent roof and I would always want to inspect and make sure the roof deck, flashing, vapor barrier, etc, is in perfect shape. 3. No problems. Don't put your tongue to it regardless of the "Triple Dog Dare"!!! 4. A picture of your home would help tremendously evaluate your necessity for ventilation. It is best to have this addressed prior to putting up the new roof and making sure that everything is working right.