While the Enterprise licence costs slightly more a month, you do get a significant increase in functionality in comparison to the Professional licence. In addition to the access you get as part of the Professional licence, you also have access to marketing capabilities, such as lists and campaigns that are exclusive to the Enterprise licenses. You will also have access to forecasting, competitor tracking and the knowledge-base.
It is also important to note that Sales Professional licences do not include entitlement to run custom applications or access custom portals. Sales Enterprise licences enable unlimited access across custom apps and portals where these are deployed in the same environment as Dynamics 365.