Sippy Cups: should I buy more plastic ones?

I have always been in the camp of plastic = bad when it comes to contact with food and water. So why is it that when I reach for a sippy cup for my son's milk I am reaching for a plastic one, even though we own 3 stainless steel sippy cups??  My current working theory is that they are a pain in the #$% to clean, but upon further inspection, they really are not! There is one extra small step in the cleaning process (I wash all sippy cups by hand because the dishwasher never really gets them as clean as I like...I live in an old rental house with old appliances!) Anyways, I'm at the point where the 3 plastic sippy cups I own have been chewed through to the point where the spout of one leaks 100% of the time. This negates the whole purpose of a sippy cup when your toddler holds it upside down...;on purpose!