A dry bounce house in direct valley sun in the middle of summer heats up, and children come out of it faster than they went in. That is the whole argument for a water slide, and it is a stronger one here than it would be almost anywhere else in the state.
As a rough line: once afternoon highs are consistently in the nineties, book the water slide. Below that a dry unit is more comfortable and less work, and it suits a wider age range — the youngest guests often want nothing to do with a slide that ends in a pool.
The set-up differs in one way worth planning for: a water slide needs a hose it can stay connected to, and the ground below it gets wet and stays wet. Put it somewhere the run-off does no harm, and keep the tables and chairs — which come with the rental — on dry ground well clear of the splash.
If the party spans a wide age range, the honest answer is often the combo unit rather than either extreme: bounce area for the little ones, slide for everyone else, and nobody goes home wet.