1)Difficulty of the mode of the examination

Pros of online over offline 

a.If you are a computer literate , it will save you some time from bubbling or marking OMR. (15 minutes can give lot of head start)

b.You can change your answer at any moment of time .

c. You can see all your answers,no of questions marked .

d. you can see the time remaining , no need to ask the invigilator .

e.They will give you unlimited no of rough sheets :P 

f.A/C in summer 

g. a college equipped with a computer lab is allotted to you, better chance of getting a good centre ( I got a LKG classroom with tiny tini benches for my boards)

Cons 

a.If you don’t practice , it will be consuming , you might panic .

while you have been writing on paper exams for ages

b.You’l keep wavering and won’t confirm some answers , once you’ve marked that question in offline , you dont need to think about it again.

2)Difficulty of level of examination 

this purely depends on your luck .

I have read some answers below that on line paper will be tough . 

But one thing I don’t like even if online paper is tougher by chance , they dont normalize the marks , just take the average *Facepalm* .

 I will just give you the trends .

 2013

I was in 11th . All my seniors gave on line , the paper they gave ( 9th) was very easy compared to offline.

Highest in our campus , a whooping 330

2014

following the trend I gave online . 

I gave it on the 2nd day , as it was considered a auspicious day *facepalm* .

The paper was very tough compared to off line and other papers according to the Internet and my whole college . I could have scored 50 more marks if I gave the 9th paper .

But the average cut off was mostly decided on offline as 70% people gave offline .

2015

I gave offline this time . Both papers were comparable this time .