Different Wheat yeasts will create differenct balances of phenols and esters. Thermperature and mashing can also affect this balance. I don't like to ferment my Weissbiers above 70*F, regardless of desired ester character or not, because the higher temps also mean an increase in harsh fusel alcohols. Though they only become a serious problem in higer gravity Wheats (Weizenbock for example) I still want to preserve a smooth finish by fermenting in the mid to upper 60s.
From my experience Wyeast 3068 ferments more towards phenolics and Wyeast 3638 ferments more towards the fruity esters.
If you desire more esters, just reduce the amount of oxygen you add in the beginning. This will retard yeast growth and in turn increase the ester production of the yeast.
Kai