I don't know what's being asked here..
Do you want to know who's voice we like better (like their tone or timbre/color) or subjective capabilities (like emotion/soul/feeling ..which is a preference based on opinions that's as varied as there are stars in the sky).
OR
are you talking about showcases the most vocal ability? like technique/range/power/agility ..technical qualities can't be argued. It's a fact that Beyonce has the the more resonant voice and showcases development and proficiency in every register of her range, which is roughly 4 octaves today. Beyonce's mastered her instrument in many ways that only a musician would understand. She's by far the better technical vocalist..as a virtuoso.
I prefer Lauryn's voice, songwriting and music in general. She's more soulful to me, while Beyonce has a different style:she entertains with showmanship, interpretive wit and versatility.. this talent and knowledge in music isn't something to scoff ..still I prefer Lauryn..she's just a more passionate lyricist.
Beyonce's voice itself holds her back..Her instrument ISN'T special in its natural state, BUT what she's developed over the years in terms of control, virtuosity, musicianship and technique puts her on a level above her contemporaries..