He shot a bit of both, but mostly negatives. I've done some printing for him and it was always from negatives. His palette is much more geared to negatives as well. It is distinct in part because of how he sees but also because of the negative and its softer rendition of color.
It's quite a difficult one actually. Most contemporary colour neg photographers shoot large format. Can't think of many notable 35mm photographers, past or present, who are known for shooting colour neg. It does tend to be slide by and large for 35mm.