Someone posted this on the boards recently:
Unfussy Female Poison Frogs Just Go For Closest Male – Phenomena: Not Exactly Rocket Science
This indicated that pumilio female darts go for whatever they found first (i.e. the closest calling male). It would appear that "looks" doesn't really have much to do with their choice then. Which I guess makes sense given that inter-species hybrids (in the tinctorius group at least), can happen quite easily in captivity, even in frogs that have different colors and calls (such as leucs and auratus).