If the 12" iBook was 600Mhz, I would again go out and buy the 12".
When you say you "need", -> You won't notice the 200Mhz difference in MANY apps you are gonna use. Personally, I wouldn't pay thet $200-$300 difference.
Also, when you are selling your iBook in the future, there is a much bigger market for the 12" ! Consider that..