It depends...
If you're lying flat on your back, then I would say MacBook, because you open it, and balance it on your chest and you don't have to "hold" it in the Air like an iPad; you just need to have a little hand on it to keep it balanced, and also you don't end up covering any speakers and the like.
If I am reading I will use my iPad Pro, as it allows for more manoeuvrability and positions without compromising consumption.
There are upshots and downsides to both for different tasks really. Just depends on what tasks you like to do in bed.