The two clubs meet in a blockbuster this weekend with former Tottenham star Dempsey a headline act.
"They were one of the most talented teams in the league without Dempsey, and now they've got Dempsey," Porter said.
"So individually they just have more than we have. And we need to make sure we're aware of that and respect that.
"He's one of the best players in the league, if not the best player, arguably.
"He still has to prove that in the league, but based on what he's done overseas he's arguably the best American player of all time. You have to put him and Landon Donovan in that category. I'm sure there will be a big debate on that. You have to be aware of where he floats and where he moves because when he does pop up he can punish you, he can hurt you."