Spears get an extra rank of supporting attacks, but not on the turn they charge. This being the case, you don't want to charge with them if you can avoid it. That is why they want to take charges. But you have run the unit with to 1 front plus 2 ranks to get all those attacks, or 3 full ranks in a horde (Hordes of course are 10 wide, but I never did this, units are too unwieldy with the 25mm bases).
HW/shield get one rank of supporting attacks, but get the parry. They can charge and still get all their attacks, but its only front plus 1 rank supporting (or +2 supporting in horde), compared to the extra rank worth in non-charging spears.
Last edition, spears by far outweighed
HW/shield, because they fought in a second rank with 2 attacks each, where
HW/S was only front rank attacking. This edition, either selection is a viable choice because of the parry on
HW/S.
My standard run (before switching to O&G) was 6 x 3 or 6 x 2 for
HW/S and 6x4 Spears, with spears as dedicated anvils and
HW/S more flexible. I liked to try to get flanks with the
HW/S guys, but they could do a lot. I also ran a lot of skinks cuz I love the little guys (and they are crazy fast), plus sallies and a life slann in a templeguard block. Was fun, but it has been a while since I played so I don't know how this stands up in the current meta.
Edits (maths error)