The wood is pressure treated pine, just like most decks are made of. Insects don't like it, but if it stays wet all the time, it will eventually rot. If you put it on a deck, it will be fine. If it is on a slab or patio, make sure there is enough slope so the water does not stand under it. My Vita (made down the road from you in Miami, BTW) has a pressure treated frame with a thick plastic bottom attached.
I would not consider this item to be a deal breaker if that is the spa you like. You are already on the right track by picking a reputable company first. Hotsprings sells thousands of spas and has lots of satisfied customers. They must be doing something right.