which is better online survey or offline survey and why .....

Depends on the kind of data one wants for his/her research.

Hello Faizan, It depends on the type of data you are willing to collect, the type of study, degree of accuracy and money you are willing to pay - cost.
While Online surveys are usually easier with less cost, they may not provide very accurate information for the following reason:
1- Programing errors/Mistakes , false setup!.

2- In case the questionnaire is well-designed and the system collect good quality data (like a questionnaire filled by registered members only), it may carry some mistakes due to false information filled in membership (people do not always fill accurate data in their accounts).

3- You may not be sure who exactly filled your questionnaire! , you never see who did.

4- I see online surveys usually get a higher bias when it comes to selected sample! In my opinion you can control the quality of data collected by offline well-designed surveys better than online ones, however you still can collect precious data through online surveys but with more complicated efforts to collect quality data.

Both type of research has there pros and cons.It depends on the research also like if you want to include more geographic , online might helps. But if you are focusing more on demographic, offline will be helpful.

With market research experence conducted both online and offline, my favour is offline, because when the editing is going on, offine is easy. Our team got experience in online research that the same person filling number of questionaires and also not just filling the questionaire as it is mailed to him/her.

Online survey is better since through online, one can evaluate people who have good background on modern technologies, communicate with people in diverse communities, can consume less time and makes the task more efficient.