This is interesting as it seems to effect all who have around the mbp/imac amount of cash to spend, here's how i see it...
they are basically the same computer, with essentially the same c2d processor, same fsb, ram speed etc etc, and at the same price points have very similar specs.
obviously the desktop benefits from a larger screen and hard drive: things not possible to include in a laptop.
whereas the laptop benefits from its portability: which is admittedly easier with an all-in-one system, but still a pain if you need to ever move with it.
That therefore is your choice, they are both great bits of kt, and will serve you well, but each have their specific targets (desktop or laptop). To put the question into context, I had this problem myself, and bought an iMac in december for uni. I had hoped that due to me only needing to move it every now and then (to and from uni each term), that i could manage with moving it: this was not the case, and i returned the iMac and bought a mbp, which serves me much better. I still use the mbp as a desktop machine (i have a 12" pb as my portable), and I have a 500GB HD hooked up to it, and hopefully soon a larger monitor, effectively giving me the best of both worlds.
To conclude, you simply need to choose whether you would move the thing...and this does not necessarily mean around town/the world, but simply that you need to ask yourself whether when working on it do you want to be restricted to a desk, or would you like to use it on the sofa? If you don't, get the iMac and enjoy all the benefits of a larger screen and HD, but if portability of any ind seems like it would be beneficial, go for the mbp.