A matter of style?

[Iñaki Garay]

Hello everyone,
while coding a spatial indexing tree, I often use a small helper
function within/9, which given three sets of xyz coordinates
describing three points, returns true when first point lies within the
bounding box created by the other two points.

This is nothing more than:

within(X, Y, Z, Xmin, Ymin, Zmin, Xmax, Ymax, Zmax) ->
    (Xmin =< X) and
    (X =< Xmax) and
    (Ymin =< Y) and
    (Y =< Ymax) and
    (Zmin =< Z) and
    (Z =< Zmax).

I ask of you this, what difference, if any, is there between using an
if clause and this function, and inserting the body of this function
as a guard?
There is the obvious code reuse when using the function, instead of
copy-pasting the body. But it makes me do this:

    Within = within(X, Y, Z, Xmin, Ymin, Zmin, Xmax, Ymax, Zmax),
    if
        Within ->
            do_something()
        true ->
            {error,out_of_bounds}
    end.

which I dislike for personal reasons.
Is using a guard more efficient than calling a function? Is the impact
negligible even when done many times? Or is it a matter of style?
Do you see a better way?

thank you for your time,
Iñaki Garay.

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