本当？Really.? Are you serious? まじで？,ガチで？ For real? ほんとい may be ほんとうに まじか　？？　This sounds unnatural in most cases.

I hope you understand English too. ほんとに is a colloquial expression of ほんとうに. And this is a very common word for most of situation. On the other hand, まじか is very casual. You can use it among young and close people, but not to adults, seniors, superiors, nor in any formal situation. I wouldn't use まじか normally, and only in the particular situation, like joking, because it doesn't sound polite. I hope it helps.