Use the Circular saw, cut the worktop upside down so that the teeth are
hitting the face rather than lifting it, cut front to back and use a
straight edge guide and line it up with a decent square. Do all that and
you
will have a nice clean, square and straight cut.

Agreed.

If you clamp the straight edge to the side you are keeping (ie. not the
waste side) then you can't go wrong. Even if you do wander from the guide it
will cut into the waste portion and not ruin your finished cut.

Sean