I would say both course are the 'best' in their own right. It all depends what are you looking for. GPEN is a 6 day intense course and deals will all the aspects of pen testing. This includes the methodology, legal issues, and a lots of theory and explanation of why and how to do stuff, and a lots of tips and tricks from professional pen testers. It does not merely teaches you tools but goes beyond telling you how to optimize it for yr situation. Its a complete course with very few gaps.
On the other hand OSCP is more on the practical side, if you can pass the OSCP challenge, that tells that you know how to conduct the pen test not just theory. This is not easy and you need to know a lots of theoretical knowledge to understand the contents of the course. Its only about 7 hrs of instructions and it assumes that you have prior knowledge of all the concepts. But the challenge is amazing
Again, the 2 courses are different, it all depends on what your goal is and how much you are willing to spend.
Hope this clears a few doubts. Let me know if you need more infi.
GPEN GCFA GCIH CISSP CISA GSEC OSCP C|EH Security+