Before there was a thought about a metal trim going around your shower niche, Manufactures made a shorter piece matching the existing tile usually 3 to 6" to be at the end of the tile installation. The edge is slightly rounded and is perfect for a seamless transition. They might not have it with larger format tiles, but can be used as a nice baseboard in your bathroom.
The question of whether to choose between a bullnose, pencil or a metal trim is completely subjective to your choosing. The benefits of using a metal trim would be the factor of saving time and labor from the installer and money out of your pockets. They come in a range of colors which is perfect if you are wanting your tile to match your fixtures. Here at ITS, we stock a list of sundry items needed to complete your project.