Onsite mixing is cheaper and a little easier in case of bad weather or equipment malfunctions.
My company, JDS Domes, LLC; attached a spiral mixer to our 2" pump so we could have bulk bags (3,000lb pre-mixed shotcrete) delivered and loaded into a 5 yard\bag silo that could also be rented by the pre-mix company. The company we used was Quikrete (for domes in Illinois and Ohio)
Look at our albums on Company Facebook Pagehttp://www.facebook.com/jdsdomes
Yes, we needed a Telehandler (reach forklift) to load the silo, but our pump is designed to run off the aux hydraulics of most (skid steers) equipment like that so it was a two for one type deal. The silo keeps a days worth of material dry and quickly at hand, we actually dump from the silo right into our mixer that can hydraulically dump into pump's hopper.