Both are good lower-body strength exercises. The choice is less about which is “best” and more about your experience, equipment and what you can load safely.
The back squat allows heavy bilateral loads and is efficient for building maximal strength
The split squat or rear-foot elevated split squat trains each leg separately and challenges balance, closer to the single-leg nature of running
Split squats usually need less absolute load, which can be easier on the lower back
Research on strength training for runners often uses heavy, low-rep work; which exact exercise is used seems less important than progressive loading
A simple combination: back squat or goblet squat 3×5 as the main lift, followed by rear-foot elevated split squats 2-3×6-8 per leg. Twice a week in the base period, once a week in race-specific phases, is a reasonable starting point.