Which Mini DisplayPort Adapter?

I have two Macbooks. One is older, the other is brand new. The brand new one did not come with a DVI adapter, which stinks. Anyway, the older Mac book's Mini Display socket looks just like the one that is behind my iMac... about 10mm x 7mm. But the new one looks like it is about 7mm x 5mm, like the one in this picture... (third port from the left)
http://images.apple.com/macbook/images/specsconnections20091020.jpg

My question is, which adapter do I use? They both are labeled Mini DisplayPort, so how do I know I am buying the right one? I want to go from this tiny Mini DisplayPort to a DVI.