My sixteen month old daughter has been off the bottle for quite some time. Since then, we have been using sippy cups, and I have come to appreciate the simplicity of the two piece cup. (We also have a three piece and a couple of four piece cups, but I’m not recommending them.) When you are cleaning cups multiple times per day, the less you have to clean the better. The more pieces the cups have, the more likely you are to lose a piece–which we have definitely done.
Some ridiculous cups have six pieces! A few of the pieces are optional, but why have a six piece cup if you can get the job done with a two piece cup? It is a bit hard to figure out how many pieces a cup has shopping online, but be diligent in finding out that information. Most sippy cups have a little rubber or plastic valve to hold back the liquid, and, regardless of how many pieces there are in a sippy cup, this is the one you will inevitably lose.
The most important thing in a sippy cup is the spill factor. Does it hold the liquid? From experience, none of the cups we have are fool proof. The little one figures out how to get a few drops out here and there, but if the sippy cup has an extra piece acting as a stopper you should know that they slip out after being dropped over and over again. If the cup is missing that valve, then your child is covered in juice or milk.
To recap: When buying sippy cups, you want ones with as few pieces as possible (I recommend two–cup and lid) for easy cleaning and to reduce the likelihood that you will lose essential pieces.