The short answer
If you manage fewer than 50 units, AppFolio Core is not open to you, because AppFolio lists a 50 unit minimum on that plan and asks every buyer to contact sales for a quote. Letting covers the monthly work a small manager actually does (rent roll, arrears, repairs, inspections, owner statements and compliance dates) on plans priced by unit band: $59, $189 and $449 per month, 2 months free on yearly billing.
If you need tenants to pay rent online inside the software, tenant screening or listing syndication, AppFolio does those and Letting does not. Choose on that line, not on the length of a feature page.