A bowler bowls 10 overs, concedes 50 runs & gets 3 wickets (its a hattrick). Another bowler under same situation bowls 10 overs concedes 40 runs & takes 4 wickets in 4 different overs. Obviously the first bowler is task was more difficult but did not the 2nd bowler is contribution more important ? Who should be the MOM?

its depend that who took more wickets of the bowler or who took wickets in early stage of the inning.. hattrick could be for tailenders. or 2nd bowlers takes 3 wickets of tailenders .

It depends which batsman which bowler dismissed.

The one getting 3 wickets even without a hattrick is liable to get the MOM is gets 3 top batsman out & the one getting 4 wickets gets 4 tailenders out.

The match situation when the wicket was ticket, how the batting team was affected by the fall of that wicket etc etc is taken into consideration when there is a close call for the award between 2 players.

Generally the Man of the Match award is decided on the basis of contribution of a player, which was instrumental for the end result.