I think the answer is it depends very much on the details of the bullying, circumstances, the kind of person being bullied, how powerful the bully is and so on. Both types can lead to suicide, the truly awful endgame.
Bullies are some of the lowest of the low and I strongly despise them.
Weirdly, I always find myself laughing or having loads of fun trying to think of snappy comebacks when I'm being "bullied" online. Plus, I know the person on the other end doesn't know me personally so anything they say or think isn't valid or important.
My initial thought is IRL is worse as the old saying, sticks n stones my break my bones but names will never hurt me goes but on reflection I see the online bullying then runs to character assassination, doxing, deplatforming, career interference etc.
Bullying is bullying and as Retro said, either can lead to suicide.
I think cyber-bullying can have more of an effect on the younger generation whereas they haven't fully developed their protection armor through experience. Though I can see how cyber-bullying can affect anyone depending on the severity. Real life bullying can be difficult, especially if you live in a small town as there is no escape from it as easily as if you lived in a large city. I believe both types of bullying are more age-related challenged as to what is worse.