If you have the 5/16 drum & tap combo, just stick with that. The only reason I used a 3/8 threaded drum is because that was all I had on hand at that time. What you are actually concerned with is the hole inside the threaded part. I personally think a 1/4 inch threaded drum is a little too small to permit good flow. Especially when shooting multiple times without cleaning, such as in squirrel hunting. I feel that the small hole would have a tendency to get clogged a little more easier than a larger one. But it is all a matter of personal preference. However, I will say that I have no experience with a 1/4 inch drum either. Sort of like saying I don't like squirrel brains even though I have never tried them......
Brian