I use primarily P3 and VMC. I also really like some of the Reaper Triads, some GW, some Citadel and VGC. It is mainly to fill out color ranges and give some variations. You may find that you like the consistency of some metallics better from one line over another. I like P3s silver metallics for the most part, with oily steel being one of my favorites. However, I prefer Vallejo golds better, with the exception of P3 Brass Balls. It is all a matter of personal preference and how the paints work with your style of painting. My friends have some differing thoughts that work for them.
Try different colors and how you mix your paints. Eventually, you will find what works best for you.