From players of different strength:
a) The stronger as he will get more decisions

From players of different aggressiveness:
a) The more conservative so the aggressive partner can raise his partner to the limit

My guess would be the more aggressive and the weaker player. Any thoughts?
(This arose when playing an incomplete Mitchell with 8 rounds and 11 tables, 2-board rounds, several sessions. Those EW pairs starting with for example boards 1&2 had East as a dealer more often than West)