This is an argument that would only be had on a Celtics board
Lebron is both the better player and has had the better career.
On top of that, the NBA is clearly more talented today than it was twenty years ago. Guys are more athletic and the game has evolved to a point where they are more skilled as well. Guys hardly shot three pointers back in the day and now it's almost a required skill to be able to have a reasonably long career.
Someone mentioned Russell too...He wouldn't even be a starting caliber center in today's league. The guys he played against in the league wouldn't be high school players in today's world.
I think it is fair to say that the training regimens are more extreme today and the game has evolved some with rule changes, but today's players don't look any better or worse to me, or any more athletic for that matter. LeBron deserves all the praise he gets, but while he has a very high basketball IQ, he just doesn't have the level of instincts that Magic and Bird had, and he just isn't as tough mentally in the clutch.
Let's keep in mind that GS didn't score after the 4:39 mark, and Cleveland scored 4 points, one of which was a desperation 3 pointer. Cleveland won by choking a little less.
And your comments about Russell are difficult to swallow. Setting aside the fact that he was smarter and tougher, Russell was also a
much better athlete than any big playing today. In 1956, he was the 7th ranked high jumper in the world. He could run a 440 in under 50 seconds.
Given how important it is for bigs to be able to show on picks out to the 3 point line and recover, he would be just as valuable in today's game.