processor-wise the difference are very small but there are other things to consider...namely expandibility and upgradeability.
with a laptop you are locked into the on board video that comes with the system and have very limited options as far as hd space.
with a desktop you can swap out the video card in a year or two and gain a nice performance boost, a real nice option esp. if you are a gamer. with a laptop you cannot.
with a desktop, you can add addt'l ide drives (nowadays most mobos have a 4 channel ide capability). with a laptop (and it depends on the model) adding a 2nd drive may or may not be possible. if so, it usually means you have to remove the floppy/cd drive. or if you decide to upgrade to a faster burner a year or two down the rd, you could move that older burner to anothe machine. this won't be not true if you have a internal laptop model.
while you can purchase laptops with dvd/cdrw combo units if you wanted to add more you're out of luck. with a desktop, have a cdrw drive, and add a dvd, and add a second burner if you like that sorta thing.
think of laptops as desktop systems modified for small size & portability. for that portability you pay more $$$, and are confined to a mid-size screen(14-15"), and as mentioned above severely limits your options if you decide to upgrade certain components.
unless you travel frequently or have limited space for a desktop (college dorm,etc) a desktop would be generally be a more flexible and cheaper choice.