The Miami Heat
After playing what I believe to be an underwhelming regular season that didn't quite reflect their abilities, the Heat finished 54-28 (this is coming after a 66-16 season with basically the same team). This is a team that was without Dwyane Wade for 28 games and coasted during most of the season. The reason I point this out is that many people think "a team that won only 54 Games with Lebron will certainly win at least 10 more games than a team that instead has Luol Deng and a Dwyane Wade losing skill by the year." That leaves them at 42-44 wins right there. You also have to imagine that last year's Heat would have won at least 60 games if Dwyane Wade had played even 75 games.
The problem is, Dwyane Wade didn't miss all those games with injuries, he missed them because the Heat thought it was their best bet to let him rest and get him in peak condition for the finals. I don't believe that was what a player as young as Wade needed, he needed less minutes and more games. Dwyane Wade wasn't prepared for the finals and he was watching critics who saw his decline pick him apart, furthering his lack of confidence. He was playing with the best player in the world and he let the critics get into his head, but this was not the player he is. Therefore, you can add two of those games back onto the win side making the Heat a 44-46 win team. Dwyane Wade will also step up in the absence of Lebron James and hopefully he can start finding his "old man game" without the sharp backlash he would get playing on a title contender.
Luol Deng is clearly no Lebron, but he is used to playing 39-40 minutes a game and playing lockdown defense all the while. He brings good veteran leadership and production to replace Lebron. Josh McRoberts also serves as a savvy veteran who can stretch the floor and get Dwyane Wade open looks.
Accounting for the fact that the Heat should have won somewhere around 60 games last year, rather than 54, you can add 6 more wins to the Heat's season making them a 50-52 win team.
I already stated in my Cavaliers preview that I believe they will be a 50 win team next season, so I think the Heat will in fact be a better team.
Conclusion: The Heat will be the better team record-wise.