So I know that these words would have the same-ish meanings, however the usage confuses me.
Which would be used in a sentence like that? Why?
I think みたい is the answer because the person B seems surprised and glad to hear what the person A said. The situation would be:
There are slight difference among みたい, そう and っぽい。 When you say みたい, it is almost true and you can hardly find difference. When you say そう, it is probably true and -ish but you are not sure for that. When you say っぽい, it sounds/seems same or resembles to the original one but you don't think it is that. You can find similar part of that but you also find many difference between them.