I use one versus the other based on bulk, reaction, forage, rate of fall. I use a jig first mainly because i like my big bites first and as far as my averages, bigger bites come on jigs.
Sometimes it is just a matter of playing with both to see which they want more that day. On a tough dragging bite sometimes they want the slimmer profile of plastics, but then on the same days they could want the more real and life like movement of the jig skirt. If i am hangin the jig up alot i switch. I just keep both handy.