Remember that Apples video editing applications, iMovie and Final Cut Pro cannot take advantage of more than two cores (and that's if you are lucky, usually it only uses one, found this out the hard way from I upgraded from a 2.8 core2duo to the 2.8 quad i7).
The dual core chips will be faster for now since they are higher clocked, but that should change over the next year when Apple releases updates to Final Cut Studio and iLife (hopefully).
The i7 and i5 will overclock to something around 3.46 i believe if not more, so you aren't really losing anything by getting a 2.8 quad i7 in dual core type tasks.