hard tackling if only you really sure your defender has high tackling skill. (>110%) i believe the higher the tackling skill is the cleaner the tackling he will do.
if your defender has low tackling skill, (<100%) it will end up much foul with cards. in this case your opponent is stronger, so, they may have good freekick taker also that you should avoid.
sometimes your striker or MC or winger with low tackling skill also will do the tackling if you play high press with hard tackling. this will end up of foul with cards
easy tackling means your defender or any player will just pressing the opponent by keep following them without tackling or just blocking, it is good if your defender has fast speed.
if your defender is a fat weight slow defender, easy tackling is not a good option.
i suggest normal tackling for slower defender with less than 100% tackling skill