It all depends on how much you've played OTB or Online when you learned chess, imo... Those that started mostly OTB find online harder, and those that started playing online exclusively, find OTB harder...
I think the main difference between OTB and Online is, online you have dots and highlighted squares to help you play, and if your king is in check, the engine won't let you make an illegal move, so you can kinda play with out worrying about too many rules (I turn those dots off. If you haven't, try it), OTB, you and your opponent (sometimes an Arbiter) are the game engine, you have to make sure you don't make any illegal moves yourself, so there is a little learning curve if you've only played online. I like both OTB and Online; I suck at both too 
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