pros of one side = cons of two side:
one side = one bank. Even your motherboard has 3 slots, if the chipset only supports 4 banks, you can't use 3 double side sticks, you only can use up to 2 double side or 1 double side + 2 single side.
one side means using higher density chips = newer chips = better value.
cons of two side = pros of two side:
nil.