It's all relative, each has its strengths and each has its weaknesses, you have to concentrate on that.
If the advantage/disadvantage question is VSAM KSDS vs DB2, that is one set of answers.
If the advantage/disadvantage question is VSAM ESDS vs QSAM, that is another set of answers.
I suggest that you treat both cases with equally and start compiling pluses and minuses for each scenario.
Post back on this thread with what you can come up with and you probably will find a lot of interesting and informative feedback and recommendations.
For your sake, I hope I'm not just "talking" to the wind....