What are the pros and cons of a lift fan versus a helicopter?
To answer your second question first:

Generally, it is always better to accelerate a big amount of air a little than a small amount a lot. Therefore, the helicopter rotor is much more efficient when creating lift, similar to a propeller which is more efficient than a jet. In the same way, more wing span reduces induced drag since more air can be used for lift creation.

The advantages of a lift fan are mainly due to its compact size:


Low mass, low inertias
Compact size, therefore easy to tuck away for fast flight
Low torque for a given amount of lift


and the disavantages stem from the same source:


Low efficiency
High exit speed, so beware of the blast.
No autorotation possible when the engine fails.
Lift acts at the center of the fan, so you need additional jets for pitch and roll control. A helicopter can shift the center of lift away from the rotor axis (within limits) by using cyclic changes to the angle of attack of the single rotor blades.