This is a simplified question to a complex issue. The medium you use (TV / Radio / Internet) has to always be linked to who your target audience is and the behaviours you want them to do.
Overall the increase in popularity of the internet means that more eyeballs are on computer screens, pulling some away from TV, but as media gets more fragmented big TV events, especially live ones such as X Factor finals and sports events are increasingly important.
With the TV it is a captive audience, people could leave the room but you can have a one way and very rich experience.
With the internet it is easier to click away and harder to get people to click on ads rather than see them, but once consumers are involved you can have lots of depth such as interactivity, games, and build up a long term relationship through things like CRM. The brand personality can come across in this way, but it requires people to make the first move, whilst with TV you can tell consumers.
Just think of modern brands such as Apple and Nike, whilst they do cool stuff online now I bet you probably think they're cool from other factors such as TV ads, events, product design etc rather than web stuff