Firewall and UTM choice - device evaluation factors

Hello everyone. I would like to ask you a question, very stupid probably: how do you evaluate whether to use a Next gen firewall or a UTM?

Also, how do you define the device model to use? Based on the number of users? Based on the type of company and therefore of use as internet traffic? What factors influence the choices of one Sophos device over another (in addition to the choice of Firewall and UTM)

Thanks a lot for the answers you will give.
MF