I like that Zorro is relatively fast for backtests, has minimalist interface while almost unrestricted functionality through coding. If backtests are all you want to use Zorro for, you probably don't need a Zorro S license. However, if you actually want to use it to connect to brokers and perform trades, then the free Zorro version is only a demo version (capital restricted to below $7000, and you can only connect to for example IB broker if you have Zorro S).