Ultimate Linux is based on Shared Hosting. And Managed Core is based on our different concept in order to deliver the fastest Web hosting speeds.
The major difference also comes in the Control Panel being offered. While Ultimate Linux comes with cPanel and LiteSpeed Servers as default, Managed Core comes with VirtualMin and Apache/Nginx Servers without LiteSpeed.
It is Suggested to get Managed Core for a High Traffic Website as it also comes with the support of cPanel in case you want to purchase it later. cPanel can be purchased at an additional fee of $10 per Month.
Something to note is that VirtualMin is really good too and a lot of people prefer VirtualMin over cPanel.