Fartlek, meaning speedplay, is often included as a shift from traditional training. The sessions are relatively unstructured, with bursts of speed/effort throughout, interspersed with easy running. A great way to do a fartlek session is to head for the hills and just run.
Interval sessions allow you to complete a lot of high-intensity work without the stress of running continually at a given pace. They also target very specific areas of fitness. The intensity and length of both the effort and recovery can be adjusted to suit the needs of the athlete but also the goal of the session. Intervals can be tough because to get the benefits of working for a short duration you need to work quite hard.
So, which is best? Well, it depends. Both provide similar fitness benefits. Intervals can be tailored to work on any aspect of fitness you want, while fartleks are a bit more ‘wait and see what happens on the day’. So I’d choose the one that appeals to you the most and adapt it to suit your goals.