This question is a answered by 2 questions, what type of property do you live in? & what is my hot water and heating demands?

A basic rule of thumb we always go by is

Flats and smaller houses with one bathroom = combi

Properties with more than one bathroom = heat only or system boiler

IF you want to find out more, visit: https://thekentplumber.co.uk/which-boiler-would-work-best-for-my-home/