Both actually. Rugby fitness training will often combine intervals of jogging, sprinting and walking rest periods. If you google the term "fartlek" there are plenty of programs available on the internet. A program which I've found effective can be done on a rugby, soccer or (American) football pitch. Start jogging down a sideline from one corner. Jog three quarters of the way around the field, then sprint as hard as you can across the field long the try or goal line. This will return you to your starting point. Then walk to the upright and back, and then start repeating the process. After a three or four of these circuits around the field, skip the walk to the goal post but (instead) do an extended rest by walking along the side and goal line to the opposite corner and start again. When I start to train new and not particularly fit players, I'll have them do two sets of three circuits. Gradually I'll work them up to four sets of four circuits.
If your a back, cardio is good. You have to to be extremely fit, but not slow. Focus on both. If your a forward, don't do any cardio, build your body mass by eating plenty and then working it off in the gym or something. You will have to build your legs using weights and that will give you the speed and stamina you need as a forward. I'm a forward, the backs lose the ball and then the forwards get it back, then the backs get all the glory for the trys. Damn them.
I used to play open side. I always hated training and the one exercise that I thought made the biggest difference was doing 400M sprints. Sure, they were hell but they really seemed suited to to the continuous running you'd do in games. I felt a punishing night of doing 15+ 400s made a bigger difference to my on field stamina than anything else.
