RealGM

While the Celtics and Spurs played different styles, both have all time great coaches, unselfishness/buy in, etc. Looking at Duncan's supporting cast is not just about the names but recognizing the high quality brand they were playing. In the case of Russell and Duncan you can also credit both stars in large part for this culture and being larger than the sum of their parts.

Both players contended long enough that they went through a variety of eras. Not a perfect comp but:

Cousy/Sharman = Robinson (old stars)
Jones/Heinsohn = Manu/Parker (line up the best age/prime wise)
Havlicek = Kawhi (had his arguable best years after they left)

Both teams tended to have the best depth

Not sure if Russell's teammates should be considered more OP than Duncan's. Or maybe it means we should consider more whether Duncan benefitted from the excellent supporting cast of the Spurs.

I'd say that neither had nearly as stacked teams as casual opinion makes it to be. The truth is that they played in deep, well coached teams but they never had that much firepower. Manu, Parker, Jones and Hondo are all great players, but compare that to teams Shaq played in, or Magic, and Bird. Hell, even some of James teams were more talented and I don't consider James to be lucky with supporting cast.

I'd posit that:

1) This is far too complex a question for a simple answer given the mass of years involved on each side, differing interpretations of the question ("stacked casts" often in reference to title teams, is it so here or are we looking at all teams each played on, are we looking at player levels or player performance levels in playoffs specifically if title oriented), the complexity of comparing casts across eras with vastly different numbers of teams in the league (and different amount of distance required from league average in terms of SRS etc to be a serious contender), is "stackedness"meant to imply the advantage to the player - in which case fit would also be involved and perhaps coaching and management structures etc

2) Whilst comps of a couple of players might inspire the idea for the team comparison you'd have to go much further down the roster (each year) to give a serious, comprehensive answer.