wow, that's a tough situation. they're both pretty important.
ob is certainly not, but gyn definitely is. in a busy ED, you have to know how to do a pelvic exam in women with abdominal pain or vag bleeding and there's not always time to get the instruction necessary for people not comfortable with the speculum exam.
on the other hand, there are things in peds that are super-common ED complaints that adult medicine alone just does not prepare you for such as neonatal fevers, the differential for belly pain in kids, RSV, flud requirements, etc.
that's a tough situation, i think you should really do both befoer your EM rotation. how many peds shifts do you have? if you've only got one or two shifts and aren't particularly interested in pedi EM, i might do ob/gyn first. if you have lots of pedi shifts or a particular interest in the field, then do peds first.