Concrete bags vs truck for slab
You should get a batch mix for this so that you can continuous pour it from end to end in a short period of time. This is the best way to assure a uniformity and even drying time for the whole slab. Mixing in small batches often ends up with one end ready for finishing when you are still messing around with mixing at the other end. 

When you get it all poured from a batch truck you then get to concentrate fully on on the troweling and finishing aspects and coverage if needed due to too hot or too cold of weather. 

Another thing that demands close attention is to ensure that the wire mesh gets pulled up into the center thickness of the slab as it is poured. Too often it gets pushed down to the underlying fill sand/gravel where is does no good. I have seen this be an even greater problem with a small mixer process because many wheelbarrow trips across the area keep pushing down the mesh and it gets neglected in the due course of going back for another load at the mixer. 

Lastly why not put at least some re-bar in the slab. It is a small cost adder to provide much greater strength to the slab to keep it in one piece.