Hi,
it's been a while since I looked at this, and the difference is probably greater now, but in previous versions of AEM, a very simplistic comparison would be that with AEM targeting, the logic you can use for specifying which experience to display would be limited to First, Random or Clickstream.[1]
With Adobe Target you can A/B test, create complex multivariate testing, implement complex rules, geotargeting etc [2] You also get Core services now so you can reuse segments across all marketing Cloud products.
Another big difference is the Analytics data you can get, with Adobe Target you get complex reports to see in detail what the effect your changes had, which helps to further optimize your site. With AEM you only report on clicks.
[1]https://docs.adobe.com/docs/en/cq/5-6-1/wcm/content-targeting.html#Configuring Experience Selection for Client-Side Rules
[2]http://www.adobe.com/marketing-cloud/testing-targeting/optimization-capabilities.html