This is a very common question - and I'm sure it could be solved with an in-depth personality study.

But PrincipalsPage comes up with some great advice on figuring out what grade is right for you:

The economy is a mess. 401k’s are being turned into 201k’s. Nobody knows where to invest.
We’ve taken a close look at the cost/benefit of a Master’s Degree and want to share our findings here.