Well, basically a downside to the neck bushing is that if you have to reduce the outside diameter of the neck "too much" it can cause some problems by reshaping the neck in undesirable ways.
I recently tried to use a .336 bushing to resize the neck on some .308 brass that has a fired case neck diameter of 0.344" . The neck ended up at 0.335" and had 0.005" of runout. When I first resize it with a .338 bushing followed by a .335 bushing the neck diameter ends up at 0.335" and less than 0.001" runout.
I've got a weird situation with the .336 bushing I have. When I first resize the neck with a .338 bushing the neck diameter becomes 0.337". Then when I use the .336 bushing the neck is still 0.337". I sent the bushing back to Redding and they said it is in spec.
In your case I would say use a .336 bushing and you're probably good.