Sigma 20mm f/1.4 DG HSM Art vs. Voigtlander 21mm f/1.8 Ultron – which one is better for travel?

Do you prefer the manual focus of the Voigtlander or the auto focus of the Sigma. I would feel that would be the main concern. They are both sharp and have little CA. Although the Sigma has slightly less it is not an issue.

If you're interested in a travel lens, I'd definitely take the Sigma F1.4 because it's an autofocus lens, which is a lot more convenient when taking pictures on the go. In addition it's 2/3 of a stop faster than the Voigtlander. It is indeed quite heavy at 950g, but assuming that would be your one lens it shouldn't be too noticeable.

If you provide us with the type of camera you're using we could provide some alternative suggestions for lenses you can take.

For a travel 21mm, one might want to consider Color-Skopar. At f/3.5 it’s darker than Ultron, but much less to carry (size- and weight-wise).