And yet it is a fact that the MBA is thinner and lighter than the MBP-13, even if that thinness and lightness are the two primary marketing points for the MBA.
I have an MBP-15 and it's a PitA to take with me anywhere because of it's size and weight and the fact that I do not require the performance nor do I need the optical drive nor all of the connectivity ports. An MBP-13 would be smaller and lighter, but it still offers components and connectivity I do not need "in the field" and there is a thickness and weight penalty therein.
I'm leaning hard towards the iPad, but if the MBA was less expensive and/or smaller, I would likely lean towards it because in the end it can produce content as well as present it and it offers more flexibility in said content presentation.
So my decision process is driven by weight, followed by dimensions and then price. It is also driven by marketing, but in as much as I am only looking at Apple products.