I guess I will stir the pot. I haven't needed a 6' tank with an oscar and even downgraded to a 90 from a 125 tall. They're bulky fish, but I have still yet to see any much over 12" in person no matter what size tank their in. Now I have seen a handful of 13"+ or atleast claimed on the internet (I may eat my words in a moment depending on who posts).
Will a 125 be better? Absolutely. Now with that being said I use a 75 as a grow out and have bigger 6' tanks stashed away that I upgrade to in a weekend if needed.