The sew in are invisible and flatter, so when the bag is closed, the opening is slightly narrower. The prong & washer type are thicker and visible.
I have found that you need less interfacing with the sew in type. The prongs have to be cut thru the fabric, and there's going to be stress on those slits as you open & close the bag. A heaver interfacing will do the trick, but you will probably want to extend it across the entire width of the bag.
So it really depends on the project. For a sturdier bag with thicker material, I would use a prong & washer type. For a lighter weight smaller project I use the sew ins, however, with correct interfacing, you can use either one.