It's pretty simple.

Do you travel?

Do you need to use it other than your desk?

If so you need a laptop.

If not then a desktop would be fine.

Desktops are cheaper for comparable systems.

If you get a laptop and are someone who keeps their technology for a while I suggest the 3yr warranty it's normally 100 bucks and well worth it because laptops are prone to break if you move them often. Also you will need to replace the battery at least once in a 3 year time span or enjoy recharging it a lot.

I pretty much get a new laptop every 2 years. I use it only for travel, and only lose a couple hundred when I sell it which is good based on what I pay and how quick computers decrease in prices.

I wait for laptops to go on sale at Dell then use my purchase program, and additional coupons to save even more. I think my most recent laptop cost $700 shipped/taxed/etc and it's normal rate was 900-1200. I got the 3 year warranty which added I believe $120 to the cost too, and a larger battery. It's one I can play games on or multi-task the crap out of since I work on it.

I use my desktop 99% of the time, and I keep it for approx. 3 years before replacing, however during that time I may upgrade a few things. I build my desktops and have for the last 15 years, and they last others an additional 3-5 years after I am done with them. Use good components and they last a VERY long time. I say 3 years because you don't even need to upgrade every year or two to continue working how you were with the new applications nowdays it's the games that decide if you need faster or better

My next desktop will be as soon as the i7 processors, mobos and ram price drops, A LOT!