you do ask a difficult question. I too would have said take the laptop, but the iPad/iMac combo does give you the portability paired with the power and screen size of the iMac.
As I have had both a desktop and laptop for the past decade, I see the advantages of each. Since you have an iPhone, how do you feel about the functionality of it, as the iPad is mostly a large screen version of that.
Whatever you decide, if you do any build to order from Apple, pass on upgrading the ram. You can do much better with third party vendors than paying Apple's premium. The processor is the only thing I would consider upgrading.
Good luck