Always sketch first. Sketches are the fastest way to explore and evaluate rough design concepts. (I’ve written about how sketches should fit into the design process previously.) Wireframes, on the other hand, are always slower. They must be made in graphics software or something like Balsamiq, which is more involved than simply scribbling on a piece of paper.

Committing a design concept to a digital format makes it less disposable. And, early in the design process, it’s critical that we designers don’t get too attached to rough ideas before we’ve exhausted our brainstorming potential. So, often, wireframes can cause us to commit to bad concepts too soon.

Wireframes should never be part of brainstorming or exploring ideas, except in a few cases where you need to collaborate with peers during early stages. (See below.)

Sketches are better for exploration, and, when no one else needs to see the concepts, beat wireframes every single time because they are faster and more disposable.