The only difference is that you have to go out of your way to feel cheated. (You have to go hunting on other forums to feel cheated.) There is absolutely no difference to someone beating me to a post on this forum, or someone beating me to a post on another one. Or, for me to have posted, them to ignore my post and to read someone else's post in the same thread and to thank them and not me (since that's what this all seems to be about, getting thanks for answering). You should feel just as cheated in that case; them deciding someone else in the thread worded it better, and ignoring your post.
It's the same thing. In all cases, you have "invested" your time, and someone else got the "reward". It doesn't matter if it's on the same forum or a different forum, the outcome is identical.