It really is a matter of personal preference.
The great thing about sippy cups is the lack of a mess (in most cases). The bad thing is you get really accustomed to them and not having to clean-up messes.
We have a 3.5 year-old and a 22 month-old. At dinner, we let them drink from a real cup (with or without a straw, we let it be their preference). That way, if they spill, it's in a controlled environment.
When we're out publicly, it's easier to take a sippy cup. At some point, we'll need to stop for our older child. You may want to look into other brands that you can take with you that don't have the straw as I've found them more prone to leaking.
In the end, try different brands - you'll quickly find what are your personal favorites. There really aren't any standards to follow on what kind to use and when. Go with your instincts - they'll almost always lead you in the right direction.