I have discovered that I write much better if I have a topic to discuss. I can happily write about just about anything but some days I can't think of anything! (suggestions welcome firstname.lastname@example.org).
It is the same as when talking to people; it's easy to join in on a conversation- even if you don't know much about the direct topic-but when you're put on the spot you're brain does this fun thing where it goes completely blank!
It's also much harder to write than speak sometimes because it requires concentration-your hands need to keep up wit your thoughts which can be difficult-especially when trying to word something just-so. There are usually more distractions when writing, in a conversation-the conversation is the focus of you and of those around you. When writing, unless you are in a library on somewhere alone, life continues, often noisily, around you.
For me, writing is a soothing, relaxing activity most of the time (bar term essays or other work on a deadline) provided I'm not suffering from writers block. However, my mother finds the sound of a pen scribbling or keyboard clicking as irritating as most people find car alarms or high-pitched d4 girls talking about boys on the train. My brothers also have this fantastic ability to make annoying noises or do mad activities as soon as I look busy or like I might be concentrating.
It seems to be a rule of life-if you have nothing to do, those around you will be busy, if you commit to something, a dozen opportunities will appear and suddenly everyone wants to talk to you. Sitting at my laptop seems to have a similar effect to heading for the door-suddenly my family are reminded of a dozen important things they have to show me, ask me or discuss with me.
There are only rare occasions where I think writing is easier than talking and it's normally when you really want to structure things. Conversations take twists and turns and tangents while writing (if you plan it) stays more or less on a similar note. You rarely start writing on one topic and then end up on a topic so far removed for the original you can't remember how you got there!
The most important time, I feel, to write rather than talk is when you are extremely emotional, it's much easier to be rational and articulate in writing in these situations. Sometimes, if I have an important talk I need to have with someone, friend, relative or relationship, I will write down all my feelings and thoughts when I'm feeling them so I know what I really want to say, and, more importantly, avoid saying.
There can be no life free from talking or from writing-the two complement and contrast one another and are both crucial, you need to be able to communicate both ways-especially in the technology age. A wild personality in person can be ruined by a dull text-persona while it is equally, if not more, disappointing to discover that the person, who is so engaging via the phone or Internet, can't keep a conversation going in person.
I don't expect everyone to be able to write, but to be able to converse via a written medium is a pretty important ability to have! And anyone who can keep up in a conversation with me is pretty good conversationalist in my book.