They're pretty similar, just the nomenclature is a little confusing.
Different metric types of instances roll up to Assessment Instances. It's a little confusing because Surveys, Assessments, and Ratings are all out of the box metric types that go there. So if you go to Assessments -> Assessments -> Assessment Instances, that's a part of the Assessment Instances table. Going to Survey Management -> Survey Instances also takes you to the Assessment Instances table. Only this time it's for surveys instead of assessments.
TL;DR: Survey and Assessment Instances are just different categories in the same table.
