I missed a lot of the discussion. What do people perceive as better/different about nighttime heart attack than about nighttime visit with style?
The idea behind Nighttime Heart Attack is that it will create internal injuries that would only appear suspicious under close inspection. The injuries will be such that it will (hopefully) be obvious to those that investigate the body that magic must have been used, and the use of the matrix will leave traces that will confirm the use of magic if any magical investigators take part in the autopsy.
Proponents of this plan argue that this will make it better for Boris's plan to pin the murder on the Lahmians, as this method will help him argue that an assassin attempted to fake a death by natural causes, thus fulfilling his request of a murder that appears like an assassination where the Lahmian assassin attempted to leave no evidence. This is in contrast to the Nighttime Visit With Style plan that involves stabbing the Tzar in the chest in a way that leaves no doubt that an assassination took place. Boris could still argue that the assassin attempted to hide the fact that they were Lahmians, but this is arguably less subtle than what Boris asked for, and it's less obvious that magic had to have been involved.
The main argument against Nighttime Heart Attack is that it's harder to pull off. To use the Matrix Mathilde must spend an uninterrupted hour doing magic on the Tzar, and if interrupted she must start anew. Detractors of this plan claim that the increase in risk is not worth the supposed benefits of its success, and argue that it is very likely that servants or guards will enter the Tzar's room at some point. In Nighttime Visit With Style Mathilde could kill the Tzar and be out of his room in less than a minute, vastly improving her odds of actually getting away without further complications.
As an answer to this objection, proponents of the plan claim that Mathilde could likely remain hidden even if witnesses enter the room. Some proponents of Nighttime Heart Attack even claim that if Mathilde is interrupted in a way that makes Nighttime Heart Attack impossible to pull off she could simply stab the Tzar and easily escape using magic, achieving an effect not unlike Nighttime Visit With Style. Therefore, Nighttime Heart Attack is sometimes argued to be a strictly superior plan: even its fail state is equal or superior to the success of Nighttime Visit With Style.
Detractors of the plan often disagree with this assesment, and furthermore some claim that the supposed success of the Nighttime Heart Attack wouldn't work out as well as its proponents hope. For evidence of foul play to be found an autoposy has to be preformed, and it's possible that Boris will simply not have one conducted. Even if an autoposy does take place, it's not entirely clear how obvious it will be to whatever physicians Boris employs that magic must have been involved. Therefore, Nighttime Heart Attack is sometimes argued to be a strictly inferior plan: it adds significant risks for literally less than no benefit.