Simply stated, NextPVR is two software components that both run under Windows ... 1) is NextPVR Recording Service runs continuously in the background and connects to TV tuners and IPTV sources and schedules and records shows. 2) is the NextPVR client that you see on screen and can use to view the EPG, to specify the shows that you want to record and to watch Live TV and recorded shows.
MediaPortal performs essentially the same functions in much the same way. Folks around here prefer NextPVR (obviously) mostly because it is simpler to use and more reliable.
NextPVR supports clients that can run on other machines and that can connect to the NextPVR Recording Service. These clients include ... NextPVR can run as a client on multiple Windows machines that connect to a NextPVR Recording Service on a single machine ... Kodi via a NextPVR add-on that uses Kodi functions for TV recording and viewing ... Kodi via XNEWA that displays withiin Kodi and has the same appearance and function as the NextPVR software ... an IOS client for iPhones and iPads (with an Apple TV client planned for the future) ... an Android client for phones and tablets (with an Android TV client under construction) ... nLite is a lightweight and easy to install client that reproduces the NextPVR client appearance and function on Windows and Linus machines (with an Apple Mac version under construction).
It might be that the deciding factors are that NextPVR is simple and reliable and MediaPortal has lost of addons and is, perhaps, less reliable. Using Kodi with NextPVR adds complexity.