Option 1: Core – This is recommended by Microsoft for saving the resources of the server. Plus server is for Admin and not for everyone is the mentality having this version. After 20 years, Microsoft finally goes back to Unix concept. But if you are not familiar with the concept and Powershell, even Microsoft offers paths to migrate Server 8 from Core to UI without totally re-installation, it is not that simple and easy, I would suggest install the UI version.
Option 2: UI – How much performance slowness you will see? My experience is “very much”. And the metro-style GUI tiles are not grouped as I expected. However, UI version is what I will suggest for your initial Server 8 trial.
Click on the bottom right corner to show the menu for Start, Setup, Control Panel, Search, etc.
Search, to find what you want to do, including all programs from the old programs menu
Setup, to change the machine, including shut down the machine