Which is a better choise? Lehigh or Clemson?

You need to earn a bachelors degree before you can apply for a PhD program, and it's considered bad form to do your bachelors and masters+PhD at the same school. You'll almost certainly go somewhere else for the masters and PhD. If you already have a bachelors, you should know that you don't 'major' in something in graduate school. Either program is great; Clemson is probably a nice place to live, and they have a partnership with BMW.