A UV-coated flash tube is generally the safer choice for portrait work. The main benefit mentioned is that it reduces ultraviolet output, which can matter when photographing bright white fabrics such as wedding dresses. Some whites and optical brighteners can fluoresce under UV, causing an unwanted bluish cast; a UV-coated tube helps prevent that.

A clear tube’s only likely advantage is cost, based on the discussion here. There’s no clear image-quality benefit from more UV in normal portrait shooting.

Ambient light only matters if it contributes significant UV compared with the flash. In a studio setup where the flash is the dominant light, the tube choice matters more. If ambient light is minimal or controlled, its effect is likely small.

Also note the safety point raised in the answers: bare quartz flash tubes can present hazards, so follow the manufacturer’s guidance and use proper protective covers or housings where required.