The verdict
For a small online store that wants to start with pop-ups and add email and SMS later, pick Privy. One product covers list growth and follow-up, and the trial lets you test with real traffic.
If your store has steady traffic, you already run a deeper email setup such as Klaviyo, and you have someone to own segments and tests, pick Justuno. You are paying for targeting and testing, not for messaging.
I'd pick Privy for most small teams that run a store. If you are not an e-commerce business at all, neither is a good fit, and a general tool such as OptinMonster is worth a look.