It would largely depend on what sort of laptop you get, some highend laptops the answer would be very little but you will be paying much higher price to get desktop specs in a laptop, check alienware's laptops
www.alienware.com
If you go for a lowend laptop you will get slower cpu, smaller hard drive, smaller screen, lower power graphics card, but for most non gamer users they would probably be fine.
But it comes down to what you want your computer to do and what you are going to use it for and if you want a bulky desktop computer taking up the entire corner of a room or would rather have a little laptop that can be put in a bag and hidden away when not using it.
Best answer, get both, can't lose then