In light mechs you move fast, fire fast and have to react fast. If you don't you die. You have to stay moving, pay attention to the mini-map, don't out-run your support, try to distract opponents, interdict LRM support, pop a UAV to show the entire opposing team on the mini-map just so your team stops peeking and reacts appropriately. Light pilots have to think fast and act independently and in concert with team mates ... turn a 1:1 into a 2:1 ... shore up a failing part of the line. Lights have a huge number of roles to play ... most of which are not rewarded well ... though a good light pilot can make 150k+ (no premium or mech bonus) ... so lights can easily be money makers.
On the other hand with assaults, assaults tend to move slower, they have a wider choice of weapons and need to have them appropriately grouped so that they can be applied when required. Assaults MUST stick with their team mates ... if they don't they will likely die ... maybe first, maybe last ... but they will die without support (unless the rest of your team was good enough to win the match without you). With assaults, the choice of approach is critical ... they must pay attention to the location of nearby team mates, they have to try to figure out not to peek one after the other into the withering fire of 4 to 6 opponents (this happens way to often) ... but they also have to learn when to push or charge ... if you have 4 more mechs with you and there are 2 or 3 opponents ... PUSH and everyone focus fire on the opponents one by one and kill them. Assaults are far more of a team player mech ... and you have to make that team work or you will probably lose. Good aim also helps. Learn to torso twist during weapon cooldown but be ready to turn back to fire. Try to watch for the opponents pushing ... focus fire on the lead element with your team mates ... if you don't have any ... you made a mistake

So assaults and lights are VERY different. They appeal to very different play styles and people. Both roles require thinking but some elements of that are different. Assaults definitely need to consider the whole team much of the time .. certainly in a different context from lights. Some folks will be amazing assault pilots ... others amazing light pilots. There aren't many that do both well since the play styles appeal to different people ... so my bottom line would tend to be different but not necessarily harder ... harder depends on the players abilities and some will find light impossible while others find assaults boring
