My main advice would be that that kind of budget seems a huge overkill for what you want to do, haha :)
But if you can comfortably spend that much I'm sure you'll end up being happy with a very responsive system. It may also be worth it looking at a higher spec soundcard like RME. Or take some money out of the pc budget to get nicer monitoring.

I would opt for the desktop 100 times out of 100
 
As has been said, you might get your laptop up to spec after several hours/days working with a screwdriver, but even then, you might find you end up fighting the system instead of enjoying it and making music - which is what we'd all like to be doing!
 
Also, if you did get you lappy running Sonar, what are you going to be using for day to day stuff? Email/internet/games(?) etc?
 
With a properly configured desktop and dual monitors, you'll be laying down your first tracks almost immediately.
 
 
What would you rather be doing?