you might consult SIR's website for details.
http://www.gov.sg/mha/sir/sirinc/index.html
BTW, it is my impression that SIR does not appreciate PR application from people who don't intend to stay for a very long time (permanent?) in Singapore. Since you are asking about the difference between EP and PR, you probably should get an EP (but you can only get if you actually have a job here and your employer will sponser you - - see SIR website for other categories that you may consider).
