I've been a happy purchaser of Maglites for years. About a year ago I bought a Fenix TK12 R5, it's an awesome little light, and my Maglites now gather dust. I'm not into all the technical details about batteries but would like to know the reasons why there is so much discussion about 18650's. I have no problems with the CR123's I use but I do hear issues with some of the sizes of the 18650's in some lights. So what are the pros and cons between the two?