Which is worse, getting rejected by your crush or seeing your crush with someone else?

I've experienced both.

I worked up the courage to ask her out after some of her friends assured me she was single, and said I had a good chance.

She was great about it, said she was flattered and let me down gently with the "oh, I would, but sorry I have a boyfriend" line. I thought it was an excuse to soften the rejection.

A week later I saw her walking on campus holding hands with a guy, and later I saw her in class sitting on his lap. Turns out she really did have a secret boyfriend for almost a month that she didn't tell her friends about, but after she said it to me, she felt she could make it public.

To answer your question, getting rejected was not as bad as I thought, but seeing her with someone else was unexpectedly worse for me.

I dropped out of that uni at the end of the semester and never saw her again, but still occasionally think about her.