OMG.. this is such a great question and i wish I would have read it about 2 months ago! I just went through the same thing, trying to find a sippy.
For my son, it seemed like it was hard for him to get the idea to tip the cup back (at 6 months) so I tried a straw-type cup and he figured it out right away! The take and toss ones are the best to start out because they have no valve (so they leak) but it is easier for the child to get the fluid out because they are not pulling against the suction of the valve. I would try those first. The ones with the straw have a hard straw, so they cant walk/crawl around with them, but we used them at meal times and it was perfect.
I have tried them all, (nuby, born free) and while he really liked the born free, they do leak, which sort of defeats the purpose.
Hands down, playtex works the best. Most are bpa free now. You can go to safemama.com and check out her list of bpa free sippy cups and just print it out and bring it with you to the store and look up every cup you look at so you can be sure to get one that is bpa free.
Good luck!
B.