On my data center, it seems like there are more WHMs. Whenever I do a ST run - I always pay attention to the raid comp and giggle to myself if we have like 7 Bards on the raid (not a single Foe Requiem was seen on that day) and such things, in addition to looking at the healer job distribution. More than often I've been having runs with 4-6 WHMs, rather than having a 3-3 setup or more SCHs. This happens quite often and I don't just run the instance once a week (bored, done with raids and helping others get their weekly item).
Now, as far as the actual server-side situation is concerned: I haven't seen a huge need for either healer more than the other. Dunno.