I can't answer your question because each of them has the potential to be very scary.
Luthor is very scary because he is acting without anyone noticing. He has a seemingly endless amount of money. He can buy anyone and anything he wants. He can set up a secret Lab and hire every scientist he wants to invent what he needs for his plans. At the same time he plays the wealthy philantropist to everyone else. In addition to that he seems pretty intelligent AND he has Nigel and Mrs Cox who are very skilled and willing to do everything as long they are paid well enough.
Trask on the other hand is evil and mad. Using him as the bad guy you could use every conspiracy theory ever invented.
It seems he can use every government agency to achieve his goals.
Both of them are scary because they are willing to use and sacrifice everyone and everything to get what they want.
In my opinion both would dissect Clark like a frog if they could. Luthors intentions could be gaining money and power.
Trask would do it to find out about any weaknesses against the upcoming invasion. He also would torture Clark to find out everything Clark knows about said invasion.
All this can happen to Superman at the hands Luthor or Trask even scarier is when one of them would find out about the connection between Clark Kent and Superman...
What would they do to Perry and Jimmy? Martha and Jonathan? Lois? Pregnant Lois? Possible Children?
"Missing Lois" gave us a little insight what could happen to a pregnant Lois at the hands of Bureau 39 ... very scary ...
Sorry I couldn't vote for one or the other but either has certain attributes to make them excellent and very scary villains.
