For sippy cups we went straight to the straw cups. There are several articles out there, and I believe i read in both baby talk, and parenting, how the straw cups are better for the kids. When you think about the sounds you make the "th" sound is not a common sound, but the regular sippy cups has the kids make their mouth have the shape for the "th" sound. Where the straw helps them keep their tongue back, as you will keep it back for the majority of speech.
I have heard as well, that using the straw will help devleop the speech earlier, and they wont' have as difficult of a time making words. i dont know if that's true or not. My daughter is 20 months, and speaks quite well compared to others at her age, but i cant say if that was becuse of the sippy cup or not.