Stick to one prep material. Take mock tests and sort out by question type. The one with lowest accuracy is your weakness say you have taken 3 mock tests and you got 9 strengthen questions out of which you got only 1 right, then you have to work on it. MGMAT tests provide reports, which are very useful in this aspect. I haven't used other test prep material. For GMATPrep you have to see each question, individually categorize it yourself and then find the weak topics. More than one test will help you narrow down easily. With one mock test, you can find out, but it might not be very helpful.

Thanks Sam

i did 33 ques of SC and got 6 of them wrong. I categorized them on the basis of concept they were testing. I realize that all of concepts are clear to me but somehow i marked incorrect answer. What should be my plan of action

2 - Pronoun
1 - idiom, verb, modifier, connecting 2 independent clauses