A word mark gives the widest protection on a word, this while a logo protects a graphic element or the combination of a word in a special font or with graphic elements. In most cases it is advisable to protect both the word and the logo. Only registering a logo has, besides the fact that graphical elements also play a role in assessing similarity of trademarks in a conflict (and could create a distance), as a disadvantage that precisely those graphical elements are often subject to change. On the other hand, a change of a logo has no consequences for a word mark.
Although often a word mark gives the best results in a conflict between trademarks consisting of similar words, relying on the logo can be decisive in some cases. This shows that both registrations are indispensable. Take the opposition on the Marshall brands at the EUIPO. On the one hand you have the trademark Franklin and Marshall and on the other side Thomas Marshall.
The EUIPO finds the trademarks aurally similar to a low agree. However, due to the identical word Marshall in both marks as well as the very similar graphical element, a visual similarity is present. And so Franklin and Marshall wins the opposition. Thanks to the registration of the logo.