Club Sailplane Real-Time Tracking: PowerFlarm, ADSB-IN, ADAB-OUT or other?

Our club is considering equipping our ships, and the ground-based operations folks, with a real-time tracking technology, so that we on the ground will be aware of club sailplanes locations and status, in order that we can better anticipate and prepare for arrivals, and schedule launches. This could be particularly useful in the case where multiple student pilots are in the air and soaring locally. We soar from a location in a valley surrounded by mountains. Line of sight Radio communication is fine for sailplanes operating within the valley, but radio communications from locations outside of the valley are limited.

What tracking technology might be preferred in this case? We are concerned that any technology that requires radio communication from outside the valley, might not work well. Would that be the case for ADSB-IN equipped sailplanes? ADSB-OUT equipped sailplanes? PowerFlarm equipped sailplanes? Other options, DeLorme, SPOT?