Optimize 360 allows you to easily run tests on the user experience of your website, such as A/B tests and multivariate tests. Whereas Optimize is well suited for smaller businesses that run a couple of tests at the same time (maximum 5), Optimize 360 makes it possible to scale this to a hundred tests at the same time. The price for Optimize 360 is custom and can be very interesting if you buy Google Analytics 360 at once.
More Complex Multivariate Tests also become possible, with 36 combinations via Optimize 360 instead of 16 within the standard version. But the best feature of Optimize 360 is probably the Google Analytics Audience targeting option. This makes it possible to target experiments through audience segmentation in Google Analytics 360.