They are used loosely as a kind of verbal decoration. At the start of a statement, they both mean "what I am about to say is literally true." At the start of a statement, "Seriously" means "I am not joking at all." "Seriously, I think we should not have candy in the house at all, because if there is candy in the house we are both going to eat it." "Honestly" or "frankly" means "I am not going to soften it the truth, even though speaking plainly might offend you or hurt your feelings." "Honestly, you need to lose some weight." As a retort, "Seriously?" means "that's so ridiculous it sounds like a joke." (Usually it is not a joke, and the speaker knows it). "The politician said that cutting taxes would boost the economy so much that it would actually bring in more tax revenue." "Seriously? Didn't they learn anything the last time they tried that?" "Honestly!" means the same thing as in a direct statement: "I am about to say something unpleasant, and I am excusing myself for being unpleasant by saying that I am being honest." "He left his dirty clothing lying on his bedroom floor again." "Honestly! He's gotten to be a pig. What are we going to do?"

A lot depends on tone of voice. - I think women shouldn't have the right to vote because they are generally weaker than men. - Honestly? Surprised disbelief tone of voice. - I think women shouldn't have the right to vote because they are generally weaker than men. - Seriously? Dumbfounded disbelief tone of voice. If I were to hear a politician say such a thing, I would reply with "Seriously?" (Dumbfounded disbelief). We use honestly also as an exclamation. - Yesterday I heard a politician say that he thinks women shouldn't have the right to vote because they are generally weaker than men. - Honestly! Who votes for these politicians? "I told you to do your homework before going out to play. Honestly! Don't you ever do as you are told?" So in common usage, if I want to react to some nonsense someone just said and stress that I find it ridiculous (so also amusing in a way), could I use "honestly"? Yes, you could use it as a question or an exclamation.

I don't think I can answer your questions about Spanish grammar. Seriously, do you really believe that? (Asked in a serious way, not joking). Honestly, do you really think that? (Are you being truthful?) I am going to try and run a marathon. Seriously? You are mad! Honestly, I have always wanted to run a marathon.

Thank you, Radziah.

yes.. honestly is the right choice