Chart.js is open source and MIT licensed, HighCharts is proprietary and you need to buy a license if you want to use it in a commercial product without their logo. Also, Chart.js uses <canvas> and HighCharts uses <svg>, I've found the former to be more performant with large datasets.
You have a good point @hakanito, I still think Highcharts is a powerful charting tool, and it is relatively cheap to set up a nice chart. BTW many companies are looking for developers with Highcharts skills
