Like anything else, there are good and there are bad shingles. If you go to Menards and buy the bargain stuff your going to be redoing them in a couple years. Ask shinglers what brand they prefer. Even some insurance adjusters will tell you which brands to avoid. If you do asphalt, it is time to strip the old ones off. Steel, maybe but think about how you are going to fasten the furring strips through the old shingles. I wouldn't waste my time with 1x4's. On any of metal roof jobs I have seen they recommended a good snow and ice dam type material be used, not tar paper. Makes you wonder how much moisture is getting under the tin. Just a 4 corner house might go on easy. If you have hips and valley's you will need to understand how to seal those up so they aren't leaking also. If dollars are a factor, the asphalts will be the cheapest and the standing rib will be the highest. Someone mentioned you can't get a 20 year warranty on shingles anymore. That isn't so. Certanteed has a pretty lengthy warranty on theirs now. I know they had a lot of bad ones just like everyone else but they paid out some money on the bad roofs and supposedly improved their new ones to meet the warranty they are now offering. Check the link.