The main difference between A/B vs preference testing is their approach. A/B testing consists of comparing 2+ versions of a product/design to see which performs better in quantitative terms. The importance here is on performance. Meanwhile, preference testing is more focused on understanding the user preference and qualitative “why” or logic behind that preferences, giving  in-depth nuanced insights. Another key difference is that with the A/B tests each participant gets to see only one version. Compared to that with the preference test you provide participants with all alternatives and you ask them to pick one.