I'm trying decided if I want to visit Messina or Taormina... Which is a better city to visit.

Also, I saw that August 15th is a holiday in Sicily... will it be possible to arrange tours on that day?

Messina is hardly worth visiting; most of it was destroyed by an outburst of Mt. Etna in the early 1900s. The city hall survived; it has a clockwork where figures parade out every hour on the hour, but you can easily give that a miss.

I would imagine that in Taormina, which lives from tourism, there would be tours even on Ferragosto (August 15), which is the day almost all Italians plan their vacations around.

But you don't need a tour to visit Taormina. You can pick up an annotated map at the tourist office and look at everything on your own, not forgetting to visit the superbly sited Greek temple for its splendid view and to sit in a café soaking up the atmosphere for an hour or two...

I was in Taormina last October and can truly say that I loved it. I was in Messina briefly, and Taormina is much more charming and quaint. Beautiful little streets, the Roman temple was impressive, with a great view of Mt. Etna on a clear day. We didn't take a tour, just wandered around on our own, and really enjoyed it. And I had one of the best canollis ever at a little bakery on the main street leading to the temple. Mmmmmmm!

Trevor,
I guess it depends on what kind of experience you enjoy. Taormina is amazingly picturesque, but entirely overrun with tourists from all over -- from about May to October.
Messina is a bustling city with few tourists. I was in Messina only a short time, and from what I had read in travel books, I expected a really seedy town. Actually, it appeared much nicer than I had expected!