Actually Pummel is far stronger than giant swing after lvl 145, but it has some kind of an issue against chaos resistance (as pummel is an unspecified physical dmg), and in general theres a screw lost in the resistance/damage types in this game.As you get stronger weapons over time, giant swing gets stronger but pummel stays the same. So the closer you are to end-game, the weaker pummel is. Which is why giant swing would be better in these cases. However if you have the +x pummel damage rings, you can make pummel superior (atleaat royal or above).
Lvl 1-145 - Pummel is quite weak unless u have silverweb rings.
Lvl 145 + - Pummel gets stronger with ancient beastbone/dl gloves + at least one ring (or brace at high lvl). I have lvl 50 Pummel, I use only mighty brace from mordy and its enough for over 5k dmg, some ppl even use more than one piece of gear.
EDIT: The boost Giant Swing gets from each skill point/str/melee combat is very low, the main dmg is ur melee dmg (each skill point adds on average something like 30, on lvl 50 its total of 1.5k dmg, melee combat and str barely add anything) and so while its useful due weps, its weaker on other boosts. It fluctuate horribly, the cooldown is 20 seconds (doesnt match shatter that way) and the casting time is so long u can do more dmg with melee by that time due fluctuation.
Pummel, gets an insane boost from str at first (just for those who wanna check the skills dmg calculator, start pummel with 100 melee combat and 10 str, then add another 10 str or a bit more), and still a very high boost from melee combat. Each skill lvl can add far more than Giant Swing on average (its roughly 4 or 5 times from Giant Swing), so it can eventually have higher dmg (but again the resists issue). Pummel has 8 secs cooldown and 1 sec casting, if Shatter would actually last for 10 secs (it lasts for 8) u could cast 2 pummels per Shatter, also, even if ur wep is slower, pummel doesnt get slower, and as a warrior ur wep wont be faster. Only issue is the drastic str plateau.