This debate is too much of an apples and oranges comparison. The NBA in the 80s and 90s was played in the paint. From 2000 to present, the game became more and more of a perimeter game. MJ spent his career slashing, driving and dunking over players who were bigger, taller and heavier than he was. Just watch the highlights (or lowlights) from the series with the Pistons. Bodies flying on every drive to the basket.
Lebron plays in a different era. Way more shooting from the 3 point line which makes everyone much more spread out. Drives to the rim are not a priority. Rules changed to clamp down on Lambeer style of play.
So, MJ's career is shorter because he started later and played in a league that was much more physical and played in the paint. If MJ played today, he would be much more like Steph Curry and would be stepping back to the 3 point line more often.
But if I had to pick who was the best, I would go with MJ. He just did things physically that made your jaw drop. Lebron is bigger, taller and is able to overpower other players. MJ beat everyone with his speed and ability to elevate like no other player in basketball history. And in the end, when it counted, MJ was able to get it done better than Lebron. Too many times, Lebron has failed to show up in the stretch and just gets frustrated with his team. MJ was unstoppable when he was on and when he was off, he pushed his team to pick up the slack.