Surface Phill.....vs Surface Untrim

Lately I have been working in between these two. From what I have seen
Fill gives you a bit more control because you can add constraint
curves and points. Untrim seems works well when you just need to patch
something up.

Fill = complex
Untrim = simple.

Am I missing anything?

 

Well, basically, yeah,..

Fill Surface with tangency (or w/o if the face is planar) can be used to
do basically the same thing as untrim but untrim is a better option if
the need is to close a trimmed surface opening or restore the untrimmed
boundaries (trimmed edges will be removed, clean sheet/face).

Fill Surface (or nsided patch or capping) has the advantage of
controlling how you want to patch or cap the opening. (trimmed
boundaries will remain, tangent edges stay)

...

 

The big difference is that Fill creates a brand new complex face, while
untrim just changes the boundary of a face to show the parts of that face
that are already there (though, to be fair, with the percentage conrol you
can extend the surface beyond its original definition).
To delete or repair a hole, I think untrim is the way to go. The fewer
faces in a model, the happier the model (espeically when it comes to
filleting and shelling).
Fill, of course has terrific applications, and I use it more than untrim.
But when I use it, I want to create something new. If the face I want is
not already there (where a change in boundary wil reveal it), I will use
surface fill.

BTW - I have really never considered the two tools to be analagous. In my
mind, the pairings are loft/fill, and untrim/extend.