Quick answer
RAC is usually safer than WL because RAC generally allows boarding and travel, while fully waitlisted e-tickets after chart preparation are risky.
What RAC gives you
RAC usually gives a shared side lower berth and a chance of getting a full berth if cancellations or no-shows happen.
What WL means
WL means the berth is not confirmed yet. It may move to RAC or CNF before chart preparation, but it is not guaranteed.
What to check before leaving
Check passenger-wise PNR status and chart prepared flag. One passenger can be RAC while another remains WL.