IDEA is a very good (and free) merge tool. Here's how to launch it:
https://www.jetbrains.com/idea/help/running-intellij-idea-as-a-diff-or-merge-command-line-tool.html

But sourcetree setup is tricky:
http://stackoverflow.com/questions/14774053/intellij-diff-tool-in-sourcetree

Please, add a default option, like KDiff3 and others.