First of all: 

I know that there's a static filter with Global: Fields comparison that would do it. But I need an exposed filter in views to fullfill my needs.

Let's say a have a table with 3 cols: "nid" | "changed" | "another datetime".

Now I want to have an exposed filter as a select list with options "none", "old" and "new".
If "old" is selected, show rows where "another datetime" > "changed".
If "new" is selected, show rows where "another datetime" < "changed".

I've been looking for a solution for multiple hours.