I think Sammy would have been a pretty good Boss . In addition to his lower key personality,he had the respect of the Unions that did business with the Gambinos,to the point that he was the one they would prefer to deal with. He also found fault with the spotlight that Gottti beamed on the Family. He said that "this thing" was supposed to be secret,and that instead of guys coming in every week ,the meetings should be at three in the morning ,with just the people involved who needed to be involved,and away from the City.
Sammy,like Gotti,also had an advantage as far as age. The Mob was starting to produce younger Bosses,and as the old generation was dying off,the members wanted leaders that they could identify with.
The only real hothead moment that Sammy had was the Fiala incident,and although it was not a smart move,it doesn't seem like typical Bull behavior.
I do think that the Gambinos would have prospered under Sammy,because as a ferocious earner,he would have the experience and personality to motivate those under him to produce.
Lastly,I think that a major difference between the two was that Gotti was a pompous asshole who saw the Boss spot as all about him, whereas,Sammy really believed in the old school Cosa Nostra,and would have been a more mature Boss.