You want the stretchier spokes on the low tension side. The idea is that the rim, under load, will push in toward the hub a certain amount, which will lower the spoke tension. If you have the same spokes on each side, the low tension spokes will go slack first.
If you have lighter, stretchier spokes on the low tension side, the low tension will still be enough to stretch the spoke the similar distance as on the other (stiffer but high tension) side. So both sides will lose tension more evenly, and not go slack on one side first.
I used to use 14/15/14 gauge spokes on my mountain bike wheels on the drive side, where tension was high, and lightweight 14/17/14 spokes on the low tension side.