I have never used a filter but would guess a grey card would be more consistent
It depend on your post production on "Consistent". I did use a white,grey,black card when I used NX for the white, mid/color, & black point and it worked quite well but you have to shoot this constantly with any change of light color or otherwise. The "expensive" WB "caps" Expodisc and a few others are very consistent and doesn't change with the amount of light. I find it to be much quicker and a lot easier to do batch WB changes.
If I did a lot of studio, white,grey,black card would probably be more beneficial. At non studio or mixed lighting even with strobes, I find the high end WB caps invaluable.
With all of this said, one thing that is important to point out, if you gel your flashes, the WB caps neutralize that as well and finds a middle point that never looks right. That is where RAW is the best choice.