A seedbox is just an online torrent client really. A VPS is a virtual computer that you can do whatever you want with (depending on the provider), one of those things includes installing and configuring rtorrent, rutorrent and using it as a seedbox PLUS whatever else you want to do with it. Depends if you want a simple or full featured seedbox. Personally I use a VPS so I can do things such as have an openvpn tunnel from my ddwrt router to my box, and use it to access foreign countries online content that is locked down by IP, I run my vent server on there and a few other small things...