"heavy weapons like moonlight sword"
What? It only weighs like 2 or 3 lbs..
But seriously, I've never encountered another player in pvp (and only ONCE in pve) using moonlight or blind (waaaaaaaaaaay low damage). Your bigger concern will be against DBS, Large Hammer class weapons (axes),and miridan hammers. A Knight shield will fall to all of these, though it's good for most everything else. Steel Shield is likely your best option overall, even though it still may get broken by a few blunt weapons (just be careful), and PFS surely takes too much str to weild. I'd say just fip a coin, lol, they are both good options.
I used to ALWAYS use the PFS until I realised it kept ruining my builds. I think "oh, 4 more points in str isn't too bad…and why not 6 more for dex to use the lava bow? And I should totally raise my faith for the Meat Cleaver…" etc, etc. It just comes down to, you really shouldn't spare unneeded points just to try and weild something if the rest of your build won't benefit from it, and raising str for the sole purpose of carrying a shield is usually one of those "unneeded" moments if all of your damage comes from dex.