in a 19 spline axle the splines are "deeper" into the stock since there are fewer of them around the diameter. in a 30 spline axle they are not as deep, but more of them. this increases the thickness of the shaft as a whole and makes it stronger (more material)
they will be easier to get and are more available, but you will likely break a 19 spline more often, so it stands to reason that you'd need spares. the way i figure it.....(this is my opinion), is that if i run a 30 spline set that costs me $350 and carry 1 spare for $175 it totals $525. if i run 19 spline and break 1 shaft every year ive eaten thru what ive saved in about a wheeling season or 2.
2006 DC Tacoma: LC-60 Hybrid front, 3 linked with 18" ORI's, LC 9.5" 3rd in RuffStuff housing, double triangulated 4-link, 16" Ballistic Fab CO's, Summit 20 Ga Cell, removable tube bed with LED's