In my opinion MTT is more difficult.
Obviously both modalities are difficult and the post flop with 100 big blinds is much more complex, however, in the cash game most of the time players are in the same circumstances, exceptions when both are 200bb deep...
In MTT there is a huge variety of scenarios. The blinds increase, this already changes the game.
At each moment you have a different number of BBs, as does each player at the table. So you have to know how to play with 5, 10, 15, 20, 25, 30, 40, 100 effective bbs from each position in different situations.
It is necessary to study each scenario a lot, it is very difficult.
There is still the ICM factor, the variety of tournament formats, regular, turbo, hyper, in addition to the issue of much higher variance in MTTs, requiring an extremely developed mental side.
One more point, tournament players have to perform for 7, 8, 10 hours straight at the highest level. This is very difficult, both physically and mentally, in addition to the social aspect being affected, as the player has to give up many things to spend 10 hours straight in front of the computer. In the long term, if this is not well planned, it can have disastrous consequences.