i don't have a special sippy for milk, but once i've switched to water, i get a new sippy. it's just really hard to get all the milk out without a trip through the dishwasher.
I use the same sippy for both, i just rinse it out right after milk is in it. The only time I really have "certain" sippy cups is when we go in the car and only because we have taller/thinner cups that fit in his cup holders.
Re: same sippy for milk & water?
I use the same sippy for both.
ETA: I thought you meant the same brand/type sippy. I use the same brand sippy, but use one for milk and one for water.