This comparison is really cloud versus desktop argued through its two most famous champions, and the argument has matured. Five years ago the honest answer was usually QuickBooks. Today it depends entirely on which of two kinds of business you run.
QuickBooks Online is the better product for the majority: service businesses, consultancies, agencies, retailers without serious stock, anyone whose accounting is invoices, expenses, bank reconciliation and reporting. It is cheaper, it needs no installation or IT, its mobile app is genuinely useful, and its reporting flexibility at the price is unmatched. Sage 50 Accounts exists for the minority with heavier requirements, and for them it is not a preference but a necessity: proper inventory with bills of materials and reorder management, sales and purchase order processing, project costing tied to the ledgers, multi company accounts with consolidated management reporting, and batch entry speed that experienced bookkeepers measure their productivity by. From £115 a month it is priced accordingly, and its desktop nature brings real costs, backups, updates and network performance are your problem in a way cloud software never is.
Choose QuickBooks Online unless you can name the specific Sage 50 capability your business cannot operate without. If you can name it, stock, orders, multi company, job costing, then choose Sage 50 and stop trying to force a lighter product to do heavy work through workarounds and add ons. The businesses that get this wrong in either direction pay for it monthly, in subscription fees one way and in staff hours the other.