Kinda both to be honest. It's not a total open world GTA style, it has a bunch of stages but they are connected. Kinda hard for me to describe right now but if you played Borderlands then you get the idea, there is a transfer section between each area and they are all interconnected.
^ That's from JSRF not the origional but it gives the general idea. I'm putting the trailer for it up in the op. Here's some of the gameplay from the Dreamcast version to show you what the game is.