although alloy and ptc (for small load)both type of resistors are in use for battery load testing; but one needs to be noted that for higher current rating (for higher power rating say greater than 10-15kw) alloy (stainless steel alloy and Ni Cr alloys) resistors are preferabl.
there are no. of advantages of alloy resistors based load banks over ptc resistors based load banks like:
Mechanical - lighter in weight, easy handling/mobilization, durable.
electrical - good of higher current ratings/power rating more than 10-15kw, sustainability, safe discharge, stability etc..