Which put more pressure on the the USSR, Reagan or Chernobyl?

Mikhael Gorbachev claims that Chernobyl was what allowed/forced him to open up Soviet government to more public scrutiny. Government misteps caused the accident, and people needed to know about it. More info.

Reagan apologists says that US pressure on the Soviet military complex caused the Soviet Union to break apart.

What do you think? Does it even matter? Why or why not?