The blue box has an additional piece which eliminates the leaking that people complain about in the yellow box variety. So you want the blue box! We have the yellow.
I am totally baffled by the names they have chosen. See to me, "classic" would be the appropriate name for the older version (with no extra piece). And "advanced" should be the newer design that includes the extra piece and supposedly decreases the leak complaints. But it looks like they did it the opposite way- "classic" is the newer one and "advanced" is the older kind. Weird. I am curious to know if the "classic" ones with the extra piece really do stop the leaks. If so, I would consider going back to Avent- DS always preferred those but the leaks were too much to deal with so we switched to Playtex Dropins. Off to read some reviews on Amazon...
I have both the 2-piece and the one piece bottles. The two piecers don't seem any better or worse than the others. I get a leak now and then from either. The extra piece is not that much harder to keep track of/wash/assemble. The thing I don't like about the two piecers is that they are a different type of plastic than the others. They are softer plastic, if that makes sense, and they pick up stains in my dishwasher. The one piecers are that hard polycarbonate material and don't get stained.
Re: Philips AVENT Bottles?
You want the blue box (I think) if we're talking about this:
https://www.toysrus.com/product/index.jsp?productId=4279461
vs. this:
https://www.toysrus.com/product/index.jsp?productId=3160420
The blue box has an additional piece which eliminates the leaking that people complain about in the yellow box variety. So you want the blue box!
We have the yellow.