Larger pin size (.029")will be more visible, especially in low light conditions. It will obscure a little more of the target, but if it's for hunting it's not much of a concern. It won't get to be a liability until you start shooting beyond 40 yards.

Smaller pin (.019") won't be quite as bright. Doesn't obscure as much of the target so allows for more precise aiming especially at longer distances.

Visibilty will also depend on how old your eyes are and what the application will be. If it's for hunting then I'd go for the larger. In fact in past years I've actually used a .040" pin for my 20 yarder and got progressively smaller as the yardage got longer.

Also take to mind that if you hunt late enough that you think you need a light to brighten the pins then you have the wrong sight on your bow. There are a lot of wrapped-fiber sights that are quite capable of being bright enough at last light without the hassle of batteries possibly going out on you. The other pain with a light is that when needed the most they often make the pin so bright that you can't see the animal beyond the sight. All you have is like a floodlight in your eyes.