Your choice. The real question, which depending on who your tour is booked through might be hard to find out, is what kind of snorkeling equipment they have? "Classic" snorkels take some getting used to. Open the mask manually to drain water out if it has leaked into your eyes/nose. Blow out the snorkel tube manually if you go under water to see something. These are no big deal, and something you will quickly get used to. However, some "modern" snorkel equipment have conveniences to make this a lot easier. The mask my mother got for my son automatically drained (one way valve) if water leaked into it. Likewise, if you dive under water and the tube fills with water, it also had a one-way valve to drain that water as well. It just made it easier. Two less things to deal with when you are trying to tread water and look at stuff.
So if you know they are going to have the classic kind and you want to get some of the new kind, you may well be glad you did. If they already have the modern ones, I wouldn't bother bringing your own. Likewise, if you've done a lot of snorkeling and those things are second nature, you probably won't need the newer kind.