Does not matter, there is not very much companion-related in the underdark from my experience.
The companions with dark vision are Shadowheart and Astarion. Shadowheart is always in my party and Astarion never. I had no problems with Gale and Wyll in the Underdark, there is a ring which grants some dark vision, you can cast several light spells, many places you fight are also lit a bit. From the companions Wyll in my view is the strongest overall class-wise, it would be a pity to leave him behind just because he is nearly blind.
