I think that telling others they're wrong for having an opinion is a bigger sin than having an opinion. Everyone is entitled to their own opinion and shutting people down because of what they say - especially when they haven't done anything wrong - is an awful thing to do, both in real life and on the internet. What makes one person's opinion more valid than another person's? Nothing. All people's views are equal and you shouldn't ever tell someone that they are wrong to believe in something.
Which is a greater sin on the internet: Having your own opinion or telling others "No"?
Right, there are only two options because this is an "either/or" question. If that's too "restrictive" for you, feel free to keep moving and don't bother to respond.
I've never heard of the word 'no' being a sin on the Internet. I'd say having an opinion would be the greatest sin on the Internet. It's the one thing that gets the most comments and responses. For example, if I were to say "I don't believe gays should get married" (not that I believe that), I would have way more responses than I could count.
Neither is considered a "Sin".
I would say a "Personal Preference" only.
From the few answer you provided, you seem to be adamant about offering solutions on Opinion only, and not wanting to discuss yours any further . . . I don't get it . . Why then interact on a social media of this sort if not to learn what other's have to think? One would thing a deviation of ideas that branch off from the main topic, would be fascinating . . . No?