As far as the difference between a desktop and a laptop, you need to ask yourself 2 questions.
First, what am I going to use this computer for?
If you're thinking about doing some heave load applications like Picture editing, movie editing, gaming, heaving data processing or anything that would put demand on the computer, then you would want to go with a desktop.
On the other hand, if you're a buisnessman/woman and are going to be traveling a lot and would like to do some of your work during flights or the like, then a laptop would be a better choice.
For this part, it comes down to how, when and where you would use your new computer.
Secondly, how much money am I willing to spend?
Latop computer are quite a bit more expensive than their equivalent desktop conterparts. On average a decent latop is going to range from $1500-$2000 whereas the same desktop will range from $800-1400.
Overall, you just need to sit down and do some thinking about the monetary issues, and the usage you want out of a new computer to decide what you're going to buy.
Its a frigging Laptop, not a Labtop!!!!