Either is fine, but functionally, front squats are more realistic.
However, everyone here should be using a routine produced by Ryan Flaherty (Nike High Performance Coach). Essentially it is a graded Hex/Trap bar dead lift followed by plyometric squat jumps. If completed correctly over time, you have to possibility to greatly increase force produced with each step, and reduce your overall run times.
Have a listen to this podcast with Tim Ferris. If you do any session per week, i would do this. I have seen some amazing results with myself and other athletes, from sprinters to marathon runners. In one athletes words “I went into Beast Mode just after 6 weeks of doing this once per week”