Both. Clear written instructions with pictures illustrating the steps in the instructable.
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+1. Tools for tasks.
burf is right
A well-written instructable includes both. Not everyone can see.
Sometimes one or the other will do, sometimes you need both.
Depends on complexity. Sometimes the pictures are simply enough, other times, reading carefully is important. Most importantly, not asking the author questions if the answer is in one or the other. :)