May be the Antares in Letojanni will turn out to be just fine for you, but I have read on internet (just google Grand Circle Sicily) about folks who signed up with GCT's Sicily then got upset after they found out that their hotel was not in Taomina.
Since GCT cares so much about customer satisfaction, and cares so much about repeating customers, they should keep customers who have already taking the step to sign up with their tour happy, and should just stick to using a hotel in Taomina. Staying in Taomina may cost GCT a bit more money, but it buys a lot of good will, more than the advertisement money spent in mailing countless color brochures can buy. (I hope a GCT Admin is reading this thread.)
I went on this tour in May 2009, it was a wondferful trip. Our hotel was Ariston, another team departing three days ahead of us stayed at Letojanni. Then both teams stayed at Hotel Garibaldi in Palermo. We met folks from that team at breakfast table in Hotel Garibaldi, we compared notes on optional tours but I did not ask if any thought their hotel in Letojanni was an inconvenience.