Hi,
If you actually NEED portability, then the laptop is the only viable option. The iMac is the better computer though - I've got the 24" and it's fantastic - the 27" only gets better (bigger screen, disk, processor, RAM).
It's going to be a trade-off between portability at the expense of features and speed, versus screen, features and speed at the expense of portability. Your call!
PS - Ignore that PC tout in the earlier post. If I'm in the market for a BMW, I'm not going to be interested in a Commodore with a body kit....
Regards,
Calx