Keep the imac. laptops wear out faster and the battery is probably half-dead. Those fancy magnetic buggers apple ships also wear out quickly. You might also be one of those lucky ones and get the macbooks that shipped with bad hard disks. Oh and the laptop cdrom seems to go quick too.
depending on the users usage patterns and eventual hardware failure this may become a factor:
It is much easier to replace or upgrade the hard disk in the blackbook.
But if you never need or want to replace or upgrade that HD then it isn't relevant.
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"When the sides facing each other suddenly realize that they're looking into a mirror, they'll see that this is the pattern for the future."
Keep the iMac. As I said, they age better. The blackbook is probably going to have something fail if it hasn't already. My whitebook from '07 has had its hdd replaced, top case replaced twice, fan replaced and the dvdrom is dead. Granted all of the replacements were free out of warranty but still.