yes the iMac is a bit faster , but normal tasks like emailing browsing , office you wont notice it , only if you do a lot of movie rendering or such things , then yes you will see the difference in speed ,but the question is if you need the bit extra speed or if you prefer the portability ,
if i would be you and had a MacBook from 2008 and if you got apple care for it i would just get a nice 21.5 inch monitor and connect it to the Macbook and wait for next year just before apple care runs out and sell it with a couple month apple care left , as you get more for it if there is a bit apple care left over