I think people feared Semenko a little bit more. Although not the Islanders who had enough toughness (Potvin, Nystrom, Gillies) and would have bowled over their own grandmother for a 5th Cup. This isn't to say Marty wasn't feared either, but Semenko was better at doing that, I think.
Marty was the better all around hockey player. Led the NHL in plus/minus in 1991. Yeah I know that stat doesn't tell the whole story, and being on the ice for some of Gretzky's 163 points that year would boost those totals but you can't be a lousy player and lead the NHL in that stat either.
Even fighting? I thought Semenko had some amature boxing experience?
I remember McSorley being considered a #2/3 guy for pretty much a decade. Where did Semenko rank? I know he was at least behind Dave Brown and, if you count Probert as part of that generation, Probert as well.
McSorley's career lasted significantly longer because he was a better player who could do more than just fight. Marty also had double the amount of fights Semenko had. Had he not been suspended for the Brashear headshot, he would have hit the 1,000 mark.
After he established himself, Marty rarely ever lost a fight as well. If you want to compare their fight cards, try these links: