Either one can be of help in both areas. It all depends on what they know, their experience levels and creativity.
Another option is a really good builder design company. They have worked with both and cannot get what they want , so do it themselves.
The bottom line is, you need to interview each company and ask those questions. Ask freinds and neighbors first, someone has done something similar and you may benefit by listening to them.
Let us know what happens, and that may be another answer for someone else with the same type question.