This makes a lot more sense. It is hard to think of "joining" which implies an additive function for filtering or selecting (both of which I think of as divisive functions). Thanks!
If this is implemented, I'd suggest defaulting to "Write my own expression" or have the UI respond to a click in the "Expression" panel by setting the radio to "Write my own expression" (definitely the better implementation of the two). I rarely filter on a single value other than isempty(), so for most cases adding an additional step of clicking the radio button would be a nuisance especially in cases where I forget to click the button and the tool shows as misconfigured or I get the wrong result from a script (potentially a lot of work to debug a script to find out that a filter was using a default simple comparison rather than a typed espression because of forgetting to click a radio button).