Good question. I don't know the answer either.
DS uses soft spouts because he is teething and that is the only way he drinks. I have 6 hard spouts and I am not sure if we will need them since DS is our first. If it comes to DS not using them I could give them to the daycare.
For what its worth, I would make sure to have a couple hard spout sippy's on hand for down the line. You may need them.
For example, my DD and her partner in crime at daycare realized last week that if you hold the sippy upside down and squeeze the soft spout, milk comes out. They made a nice little puddle of milk on the floor of the daycare playing that game. DD also played the "milking the sippy cup" game all over my living room carpet and her bedroom rug as well. Eventually I smartened up and stopped giving her soft spout sippy cups. That pretty much solved the problem of puddles of milk everywhere.
My best friend's DS is a month older than DD. She had to switch to hard spouts because he kept chewing through the soft ones.
You may run into the same situations eventually and need the hard spouts. Keep 'em if you've got 'em.