In Japanese, there are formal Japanese and informal Japanese. Usually Japanese people use informal Japanese in conversations. So ほんとis informal Japanese. You can use in conversations. But if you write any sentences, you have to write ほんとう. Because we the character of ほんとう is 本当 that is read as ほんとう. So ほんと is a kind of slang. I don't know that you can understand my explanation. If you can't, and you have more questions, send me a message directly.