there is defference in qestion like tough and easy in online and ofline mode in jee main and advance
jee main is option online/offine but jee advanced is only offine.
I preferred offline mode.One can easily look the question and solve it below the question,unlike the online mode where one would have to first look on computer screen and then copy the values on paper carefully.
Jee mains is a national level exam in which there is no difference between the questions asked in online and the offline mode .there are same difficulty level questions that had been asked their.
Be prepared well for the all topics which you have covered.
Also I would suggest that you should try some mock tests regarding jee mains in offline as well as online mode or the mode which you prefer.
All the best
Shubham Goel (DTU)