been my dilema too
i am used to the size of the 12" and have used ibooks for the longest time... as my main computer!
they have been terrific and i have even been happy with the screen. as a designer, the only times i feel i suffer, is using something like illustrator or archicad. (my eyes are going a bit wonky, but that is not just a screen size/quality problem, but also a factor of the amount of time i use the computer):
even quark, photoshop and formz are fine on the 12". no problem with ms office, myob, safari etc.
i do not do any video editing etc, so no views there.
as this seems like it will be your 'only/main' computer, i would say from my experience, that the 12" with acd will be amazing and the best solution by far.
unless:
-as said by someone prior, you will be away from home a lot, then the 15"er could make sense.
-or if you are like me and like to be mobile around the house and not feel like being stuck to sitting in front of a stationary screen, then, the 15"er could also make sense.
i am probably going to buy a new pb now. i am pretty set on getting the 15"er, as i enjoy using the computer all over the place at home. i am also someone who does not like having the 'traditional' desk arrangement for my work.
i am just so used to the convenient size of the 12" that i am terrified of having a more cumbersoome 15". however, the new specs of the 15" for me, look terrific - primarily the improved brighter and shaper screen (about to see it in the flesh later today).
and the speed will be just wonderful, as i am coming from a 500mhz.
...and that should help answer your second question about speed. will you notice a speed difference between the 12" and 15"? NO!
so basically the bottom line could be viewed this way:
as you main computer, i would suggest the 15", as the screen and specs are a step up from the 12". UNLESS, you are using the 12" together with an acd, then the 12" makes great sense.
whaddyathink?