Get the laptop; considering the use you have said you will put it to a desktop's extra performance is going to be unnecessary. As for the screen size; a 15 inch laptop screen is perfectly adequate, as is a laptop keyboard for typing.
As for the ease of upgrading various components; why worry? You'd really have to have some very specific requirements to need more than what the average, even cheapest, laptop offers these days.
And I use both desktop and laptop computers all day in my engineering/computing job, but opted for just a laptop at home.
Finally, I'm typing this lying on my back in front of the TV using my wireless broadband connection - try doing that with a desktop!