Any set of the angles will do (just like returning q or -q is acceptable as well). If you want the other quaternion representing the Eulers, negate it.

I guess the reason for ambiguity is because of the call to asin (which returns angles only in a 180 degree (or pi radian) range), not the full range (because of how inverse functions work, they can only return one value). ~~Try adding pi on to the return from asin, you will probably get the other quaternion instead...~~ (EDIT: asin(x + pi) != asin(x) so ignore that ;), the formula is asin(x + pi) = -asin(x), I think [too much beer])

(atan2 does return full 2pi radian range though).

I wouldn't worry about it though. Note the trig functions don't really have inverse functions (since a function can only return one value in the strict mathematical sense of the term), asin is a restirction of the "inverse" sin funtion so that it does have a unique inverse.

EDIT: Try thinking of it like this instead:

why doesn't sqrt(-1 * -1) return -1? Same deal.