Well, I'm not in the concrete business, but I do have a degree in civil engineering, so I'm kind of particular about my concrete. If the ground is undisturbed (not fill) and has a high clay content, you can remove the grass and pour right on the dirt. Heavy loam or sandy soil needs to be excavated about six inches and a bed of compacted gravel put down. Never move dirt around and then try to pour on top of it. Even if you compact it, it will settle. Four inches of concrete is minimum, six is better. The cost increase in marginal. Welded wire fabric will keep the slab together when it cracks. Concrete is going to crack, nothing you can do. Some people like fiber reinforced concrete, but it is better for high traffic areas where the top gets a lot of abuse. Wire gives the whole slab integrity. If you live in an area with a heavy freeze-thaw cycle, order air-entrained concrete, it will keep the top from spalling.
I notice you are putting your slab up against your house. I would not advise trying to pour it level. A very slight slope, maybe 1/2 inch per 10 feet will keep the water away from the house, and keep it from pooling under the tub. Your tub won't even notice that slight a slope.
Don't be temped by fancy "stamped" concrete. It is more important that the tub be evenly supported by a uniform surface. A medium "broom" finish in the direction of the slope will give an attractive, non-slip surface. Cover with plastic, but pull back to mist with water to keep the surface damp. Cure for two weeks before putting the spa down and filling, don't skimp. The only thing about concrete harder than doing it is doing it over.
All of this advice is worth exactly what you paid for it. Find a local concrete professional and ask their opinion. A company that employs a registered professional engineer is a good sign. A good place for a referral is one of the "backyard" stores that deals in storage buildings, gazebos, decks, grills, and sometimes, spas.
