Most people prefer to buy a stand-alone modem and use it with a separate router. You will achieve faster WiFi speeds by using a separate router and modem. Plus, it is less expensive to replace one or the other than it is to replace an all-in-one unit if something fails down the road. However, two separate pieces of equipment take up more room and maybe more troublesome for someone with little tech skills.
A WiFi modem is a two-in-one unit as it combines a modem and router. These are easier to setup for the less tech savvy person, and take up less shelf space.