Which exam should I give online or offline in jee?? Is there any disadvantages in online exam???? Like loading errors of questions and problems like buffering???

No no dear there is no such problem like buffering or loading error in Jee online. You will be provided with exact 3 hours time as per as the process.

You will not face any such issue related to technical problem with computer or as such and if you face too there will be a person already present there. Moreover as exam time is counted by software you will get full time

Also you will get 1 week more for preparations for online exam. So in my opinion it is is a good choice