Mmhhh, the difference? Everything .... they aren't the same kind of object as one is a filter and the other is a variable ...
Add some context around your question to get a better answer (I can imagine why you ask but it's up to you to define the context of your question)...
There is no much difference. Either way it will work. But defining a presentation variable will have certain advantages such as using in column formulas and in complex filter conditions(such as convert to SQL filter).