SPAY!
I always wait until after the second heat for my girls. You want the hormones for normal growth, but once they are grown, do the spay....I've recently had a bunch of friends that just were too lazy to get the surgery for their dogs, but ended up with REALLY ill dogs and super $$$$$$$$$$$$$$ surgery from pyrometra. Scary scary.
Here's the article about why wait until grown...
http://www.caninesports.com/SpayNeuter.html