I think there are a lot of good points in favor of LeBron, but I still think MJ wins out, if only slightly.
> MJ had a much better supporting cast
To what degree were Scottie Pippen and Horace Grant/Dennis Rodman "much better" than Kyrie Irving and Kevin Love, or Dwyane Wade, Chris Bosh, and later Ray Allen? Don't get me wrong, I'm extremely bullish on Dennis Rodman, but none of Jordan's teams ever had an All-Star big man, and consistently defeated teams that did (New York, Utah, Seattle, Detroit). That was especially abnormal in the late 80's and early 90's.
> one of the greatest coaches of all time
This is the biggest point in LeBron's favor, though Eric Spoelstra is a little underrated. There's a lot of overlap between the greatest coaches of all time (Popovich, Jackson, Auerbach, Riley) and the greatest players of all time (Duncan, Jordan, Kobe, Shaq, Russell, Magic, Kareem), and LeBron is the biggest exception to that.
At the same time, the pre-Phil Jackson MJ was dubbed "God disguised as Michael Jordan" by Larry Bird and was clearly a wasted talent in search of a coach, team, and system that would make the best use of him. And the biggest criticism of Phil Jackson is that he's never succeeded without having either Michael Jordan, Shaq, or Kobe Bryant playing for him. Aside from the frankly bizarre end of his tenure as GM of the Knicks, Phil Jackson's greatest strength was the ability to manage the egos of passionate, erratic superstars.
> and didn't have to play the most dominant team ever (Warriors) for 4 consecutive Finals
Michael Jordan was the centerpiece of "the most dominant team ever" for two sets of three consecutive finals.
(And frankly, if you want to talk about "the most dominant team ever", you have to look at the 1957-1969 Boston Celtics, who won all but two titles during that span.)
> The Bulls won 55 games during the first year of MJ's hiatus. The Cavs won 19 games the year LeBron went to Miami.
This is a pretty good point. I think removing Michael Jordan from the league entirely led to greater parity overall, and the 1993-94 season had a lot of indications of that (three conference semifinals going to seven games, the 8th seed in the West knocking out the 1st seed in the first round, only one team below 20 wins, only one team above 60 wins). The Bulls without MJ were worth a second round exit, which isn't nothing, but a Cavs team with Kyrie and Love would have done just as well if not better, and the 2016 Heat also bounced back to a second round exit two years after LeBron left.