Basically the small-rock-pump mixes have more sand and less #8 coarse aggregate compared to the standard small-rock mixes.  This just allows for the concrete to go through a small hose easier.  Because of the extra surface area generated by the increased sand, extra cement and fly ash is required.  This extra cementitious material increases the water demand and slightly increases the admixture requirements.  Due to the extra water, small-rock-pump mixes shrink more than standard small-rock mixes.  So, if the concrete is not being pumped, the standard small-rock-mix is preferred.  This reduces the cost and shrinkage.  However, if the concrete is being pumped, the small-rock-pump mix is preferred.  But remember, a small-rock mix shrinks more than a large-rock mix, which means it cracks more and costs more.