Hi ktlou
As a scuba diver, I would recommend silicon - its usually more expensive, but does last a lot longer. Rubber masks are good to start off with but break down and become 'sticky' over time.
Another point, as I'm not sure from your post if you are intending to buy over the internet, but I would advise you to go to a dive shop and try on the masks. They are all different and the only way that you can get a good fit is to try on loads. You should be able to get a good seal by just inhaling when you have the mask on (you dont even need the strap on for this - this just keeps it secure while you are in the water) If when you inhale, you get any leaks around the sides, try another.
Enjoy your hols and the snorkelling