ScreenZen's core positioning is free. Its official site describes the app as free and donation-supported, which means its delays, limits, schedules, goals, and stricter blocks are available without a subscription.
Opal's free tier is intentionally limited. It covers 1 rule, basic timers and soundscapes, app blocking, and same-day Opal Score. Reaching Hard Mode, more rules, or full analytics requires Pro, priced at $9.99 weekly, $19.99 monthly, $99.99 yearly, or $399 for a lifetime plan.
That price difference is the single biggest factor in this comparison. Opal's yearly price alone is close to ten times ScreenZen's cost of nothing, so the decision often comes down to whether Opal's specific Hard Mode and analytics justify that gap for you.