Unless you are a gamer or work on CAD or something similar - buy a laptop - Office Depot has some for $400 or so - I have a range of computers from desktop to laptop of various ages - my 3 year old laptop runs what I use just fine...
Not sure what Adam's response will be, but from what you are describing - an inexpensive laptop would likely fit the bill. As far as the rest, buy the extended warranty if you need insurance/peace of mind. I have 3 laptops, ranging from 1-4 years old and have not had to repair them. I am an old IT guy (20+ years of experience) and I don't think I will ever buy another desktop.
As far as stolen...just be situationally aware - or keep it at your house. In a locked cabinet.
Well, if that's all you want you only have to make your decision based on ONE thing-how badly do you want portability? If you want that badly then by all means a lap top is the way to go. However they are hard to repair (no do it yourself!) and upgrade. If you got a bare bones desk top and decided you wanted more power, storage, memory, etc. it's easy to upgrade those things and fairly cheap as well.