I use a Sony PMW-200. I thought about purchasing a good quality 18% gray card for my white balancing. However, I was also recommended to try the Vortex Media WarmCards 3.0 instead. I film mainly indoors in a green screen studio. However, we also do shoots outside (sunny bright skies, snow glare). Any suggestions & recommendations?
You should balance to a white card, not gray. The warm cards are to cheat the camera toward red (warm) by removing some blue in the picture. You can do this in your camera profiles, too by adjusting the color matrix. This might be helpful.
Nothing wrong with using a true gray card for white balance. the process merely requires a target that is devoid of color! An advantage of gray might be that it's probably less susceptible of reflecting another strong color in front of it?
For your green screen shoots, if lighting with tungsten balanced photo quality lights, preset 3200 is a good choice.
I always recommend flipping back and forth between preset and the AWB while observing on a properly adjusted monitor and choosing the best look. It might be counter intuitive but still correct.