As said above definately get two, it makes their transition to their new home easier if they have a litter mate with them, plus if you will be out at work all day they have company.
Gotta disagree on that one.
My 2 cats arrived from different breeders (it was planned that way). Neither of them had any problems settling in.
In fact, a friend of mine who bought his kittens a month before me, got 2 kittens from the same litter and is now having no end of problems - his male cat is spraying and its getting to the point that he might have to give him up.
Another advantage of getting cats from different litters/breeds is that you get 2 cats that are very individual. When you get 2 cats from the same litter, they tend to look the same (see Marf's pics) and grow to the same size. Their personalities might differ, but in general, they are very similar. If you get 2 cats of different breeds or different litters, they will both be physically unique.
As an example, my Egyptian Mau is not affectionette. If you pick him up, he wants to get out of your grasp immediately. He doesnt sit on your lap, which is typical of the Egyptian Mau. He is a small cat, but is lightning fast. He breed characteristic states that he should be able to out run any other breed of cat in the world.
My Bengal on the other hand, is substantially larger, heavier and slower. He is like a giant in comparison. He is affectionette and doesnt mind being hugged or picked up. He doesnt have the lightning fast reflexes of my other cat. He is also very vocal.
Both cats have individual voices and without seeing them I can tell which cat is calling. Their behaviours are also very different - eg. Reflex (Egyptian Mau) is a great climber and so prefers to sleep at the highest points in the house. Mr Bengal is bigger, lazier and fearless - to this end he tends to sleep wherever he wants, eg. the floor, and doesnt crave the safety of a high vantage point.
In my opinion, its always better to get 2 cats from different litters.