both are fairly terrible but I'd go with the spitting one D:.

I think I'm going to vote for the spitting because both of you feel awkward, whereas the snot thing they don't realise until later and then they curl up and cringe for hours in embarrassment wondering if you noticed (which you totally did, so did everyone on the train, the lollipop man at the crossing by the school, all the kids at the school and their parents, their elderly neighbour Mrs Morrison and their pet cat)