These are difficult species. The photos of Rufous-bellied Niltava at OBC Images seem to indicate that on this species the orange underparts are uniform all the way down to the vent/undertail coverts. Fujian Niltava shows an obvious contrast between the richer orange breast and the paler vent. Perhaps the contrast between the vent and breast is more important in terms of ID than the breast/belly contrast.
The photos of the King's Park niltava - although they are variable because of exposure etc. - seem to show a contrast between the breast and vent, suggesting that this is more likely Fujian Niltava.
David
