I would suggest that you use a comprehensive approach with a solution provider. Let them use an implementation partner if necessary depending on the local availability of resources.
The solution provider has better access to his internal development resources to see through enhancement requests that will be essential for the succcess of the implementation. The breadth of project implementation experience also exists within the professional services organizations of the solution providers. Noworries about that too. You can also leverage their internal experts on relevant verticals, depending on the size of the deal and the relationship.
The only plus point that the implementation partner could have brought along is the depth of change management and project management expertise. The lack of that needs to get compensated through high involvement from your user team and careful choice of steering committee members to see through the implementation successfully.