Smoothness. More cylinders equals smoothness.
Q1: Not so easy to answer. Keep in mind that we're talking higher efficiency for lower number of cylinders. You can have less fuel consumption for the same power, or more power for the same fuel consumption.
Q2: Again, not easy to answer. Different number of valves mean different combustion chamber shape, means different combustion. However, generally, more valves produce lower pumping losses (lower power loss due to suction of air into cylinders), but increase valvetrain inertia (causing more power needed by the camshafts). It could balance out, or it may go either way. There's no easy way to say other than do calculations for specific cases. All Europeans and Japanese go for the multi valve to reduce pumping losses, while the Americans use pushrods to decrease valvetrain inertia and friction.
Compare two V8s (assuming constant rpm):
DOHC:
32 valves (inertia), 32 valve guides (friction), 32 followers (friction and inertia), 4 camshafts (friction), 1 or 2 belts / chains (friction)
SOHC (3 valve / cylinder):
24 valves (inertia), 24 valve guides (friction), 24 followers / rockers (friction and inertia), 2 camshafts (friction), 1 or 2 belts / chains (friction)
OHV:
16 valves (inertia), 16 valve guides (friction), 16 pushrods + roller followers (inertia), 1 camshaft (friction), 1 gear (friction).