I do both, but if I had to choose, I'd say foam rolling is more important for me. It helps loosen up my IT band and calves, which get tight after long runs. Stretching is good too, but I feel like foam rolling gets deeper into the muscle. Give it a try and see what works for you!
Great advice! I'd add that it's also important to practice running on different types of terrain, like trails and hills, to get comfortable with the varying surfaces. And don't be afraid to walk or hike if you need to - XC is just as much about strategy as it is about speed.