I print on Kodak metallic paper a lot. (Endura). Some shots work really well on it, some just don't. If the shot has lots of saturated colors, or large dark sections, then printing on the metallic paper works. Of course a test print is cheap and if I'm unsure I just print both.

Now for directly on metal, that's a bit different. It is expensive enough that you want to get it right. I've only done it three times and all three times I chose a photo that I had already printed on metallic paper and I was happy. I just wasn't willing to risk it!

Now the next thing you need to know is that printing on metal results in a frameless piece. This is actually pretty nice in a modern setting but they are much smaller than you would think. For example, I print a lot of 10x20" prints. Put that in matted frame with a 2" border and a 0.5" frame and you have a 15x25" piece. This is a great size on a wall. But print a 10x20" on metal and, well, it is smaller than 15x25! So you are tempted to go bigger. My biggest is 20x24" on metal. Looks great but man, it wasn't cheap. It was for a juried show so I didn't mind the expense, but there is no way I can stock the gallery with pieces like that, too much upfront money!