If Trump is still President next year, McCarthy. He's called McCarthy "my Kevin." McCarthy has done a massive suck up to Trump. In this situation the White House typically has some weight in recommending who becomes the House Speaker. (Some, not totally). 90% of Republicans still like Trump and distrust the GOP in Congress, so you can expect Trump's views to be important, unless there's extenuating circumstances that prevent him from being influential in the race.
It might be a different dynamic if the race is for House Minority Leader, because you need to just have a majority of the conference instead of 218.
I'd say McCarthy's *more* likely to lead the caucus if the GOP is in the minority, because, as you say, you only need majority support in the caucus to be elected Minority Leader, and I think he can get that. If the GOP actually holds on to the House though, then the whole thing is more unpredictable, because a handful of rebels in the caucus can hold the process hostage.