(Or the other way around. I agree with Marco on that one, I think it's confusing.)
In practice there is 90% overlap between these terms and both denote the tertiary source you became aware of the link through.
The difference when it exists is that "HT" is less likely to add anything you don't see in the source whereas "via" will sometimes be worth clicking through to see that person's insight.
Hat Tip:
A -> B -> Me
B added nothing, you should visit A for the source. Goodwill link to B, you should check out his site.
Via:
A -> B -> Me
B may have added something. You should visit A for the source and if you have time B might be worth it.
Flickr photo (ori) -> example.tumblr.com -> my blog "photo via example.tumblr.com"
Youtube clip (ori) -> John Doe on FB -> your FB "youtube clip via John Doe"
so 'via' is the place where you clicked repost/share not the original content
I always interpreted them as mostly meaning the same thing.
"Via" should not mean the original source because it means "through" or "by means of".
I don't know what hat tip is but original sources are important and the least someone can do when they rewrite your content is offer up a direct link to you. The whole rewriting content thing is kind of annoying in the online news sites though.
I still don't know. What is this hat tip?
Frankly I never understood the concept of 'stealing' links. Once ago a friend on fb got very offended that I posted a link which was on his fanpage without 'via', like everybody is obligated to advertise random sources for stuff that already have thousands of views/shares. Maybe back in the day linking was worth something and I remember all those linkblogs like halfproject, surfstation, k10k, etc., but today with all those tumblrs and other tools to aggregate, suggest, reshare, cross-post, I don't see any particular value in clicking 'share/repost/+1' that needs to be credited. I'm with Marco on this But regardless of how much time it takes to find interesting links every day, I don’t think most intermediaries deserve credit for simply sharing a link to someone else’s work.