I've used both. firefly-iii is a fairly good, easy to maintain and host, web app. It's not as good design or UX as Actual - especially since firefly-iii has no mobile experience to speak of. Acutal had great mobile apps. firefly's budgeting interface is a bit of a UX mess.

It’s far less flexible of a system than firefly-iii.

That said it actually (lol) functions well with any amount of historical data. Firefly-iii slows to a crawl with just a few years of data.

Actual handles nearly a decade worth of data (imported from YNAB4) with ease.