I think this relates to how old you are. I definitely remember it being used for Treets and I thought it was used for Minstrels later. I also did a search and found the M&M reference. In this day of huge corporations, you'll probably find that all of these product brands are owned by the same company and they've carried the slogan over to a newer product. M&M's were the replacement for treets in the UK some years age. I wish they'd bring the original treets back. Far better IMHO.
Minstrels definitely used it, and I do remember someone else using it later when I thought "they've ripped that off Minstrels!". But I don't know what the other one was. M&Ms seems more likely because I remember them being launched over here, as opposed to Treets which go waaaaay back.
All the brands mentioned (Treets, M&Ms, Minstrels) are owned by Mars Inc. I am fairly sure the first one to use the slogan was Chocolate Treets, long before M&Ms and Minstrels existed. Chocolate Treets changed their name to Minstrels, amd Peanut Treets became Peanut M&Ms. Confusing or what!! M&Ms are obviously a Smarties copy. I think Minstrels now use Galaxy somewhere in their name, which is another Mars brand.