Well, as the lore goes, they don't wear armor (or more importantly, the gutplate) because of their faith in the powers of their god. The only reason why we're able to pull of the "glittering scales/fencer's blade" combo is because of an oversight in the book; because they're allowed to have ironfists, which are shields (and therefore a type of armor), it opens the opportunity to have magic armor. If we were to follow the lore more aptly, Slaughtermasters wouldn't have any armor or armor-like substances on them; their own billowing layers of fat and filth, alongside their steadfast devotion to the Maw, would be more than ample sources of defense for them.
So, as to what they would "wear", persé, it would probably be at the most an apron (and then pants, because for the love of God you don't want to imagine what Ogre genitalia would look like on a being that has rolls of fat all over them {See Peter Griffin}). However, a completely unarmored and full exposed
upper torso would be adequate clothes for them... You could also, perhaps, model various chunks of flesh and offal as a sort of "cover" for a Slaughtermaster, if you wish to go through that effort. Which would look awesome, btw

As for their looks in comparison to a Butcher... Well... I'd say, they'd be fatter, for one. Probably having more bits around them, maybe a larger, uglier looking cleaver or knife. I don't know how to describe it, though... they'd have to look more... legendary... I guess? Kinda like how the archmage (like Mr. Cthulhu pointed out) would have more flowing robes and maybe a larger hat than a regular mage... The Slaughtermaster would look more, idk, meatier, nastier, crueler perhaps?
My particular Slaughtermaster model had the benefit of getting kitbashed at a hobby shop birthday event; so, I ended up giving him a horse, whose head he holds, and whose base is adorned with the hacked up carcass. Oh, and I meant to mold an apron or satchel that would carry the horse's legs. They aren't good eats, but for the sake of a good Toothcracker cast, nothing would be more sinewy and crunchy than a horse's leg

33 Gnobs! Over 3 Packs of Trappers! Woot!
Gibblet, Nibblet, Crunchybit (from Slaughtermaster achievement)
New book WLD: 20/10/2 - Yay tourney wins and random game win!