For owners/breeders who have a mare in foal, what gender are you hoping for?

Slightly depends on the state of the market and slightly depends on the quality of your pedigree J. Daff.
The stock answer would be colt, everytime. However there is plenty of evidence to suggest that when the market is tricky, and buyers are scarce, long term players get more involved looking for fillies with strong credentials as they then have an asset to hold long term.
Foals/yearlings with a pedigree of limited established appeal (sire or dam) must be very good looking and must be a colt.
However youngsters with a top established female pedigree but an unfashionable sire will probably sell better as a filly as again, their appeal is not just a fleeting one.
Hopefully others will tack on some other considerations here.