For any usage, the most important thing is to find a pencil you like and feel comfortable using. Some people prefer to draw with pencils intended for writing, and some people prefer writing with a drafting pencil. As long as you are happy, don't worry too much about the intended purpose of any particular pencil.
Writing
Pencils for writing usually have a conical surround to the lead, which is stronger, and more resistant to bending. The lead is sometimes sprung, to absorb impact when writing firmly, and minimise snapping. Other important features may include...
A comfortable grip, especially if you're likely to use the pencil for long writing sessions.
Small size may be important if you want to write anywhere, and need the pencil to be pocketable or fit in a pen loop in a favourite notebook.
Our Favourites
Faber-Castell Grip Plus Faber-Castell Grip Plus: a nice chunky rubber grip, and a nice thick twist-advance eraser.
Pilot Birdie Pilot Birdie: handy little pencil for notes anywhere - fits in even quite small pencil loops. Unfortunately, not pocket-safe.
Pilot The Shaker Pilot Shaker: the Shaker pencils are loved by some students - the shake to advance mechanism means they don't lose track of the notes whilst hurrying to click out more lead. The -X version has a nice big rubber grip, making it more comfortable for doing lots of writing.
Office Use
Most office use will come under the general heading of writing, but companies are usually less willing to pay extra for something that feels nicer. If you get to choose for your office, you need to get the best value, but you also want to find the most pleasant pencils to handle if you're going to spend a lot of time using them.
Our Favourites
Pilot SuperGrip Pilot Super Grip: the cheapest mechanical pencil in our current range, but works well, and the rubber grip makes it comfortable for extended use.
Faber-Castell Grip-matic 1377 Faber-Castell Grip-matic 1377: another great value pencil for general use - fantastic features for the money, and still has a quality feel. The automatic lead advance means less stopping to click the button, and the twist-advance eraser lets you cover up more mistakes before the boss sees them ;)