This is my two words on the topic:
Navision developed as very structured, integrated and very generic solution that very easy to modify and tailor to specific customer needs.
GP developed as GL system with multiple add-ons. (GP was buying different solutions from different solution centers)
As result: if you can find Vertical solution tailored to your type of business you can pick any system (not only Navision, GP, Solomon, Axapta, MAS90, SAP…). It is always better to find Vertical.
If you can not – go with Navision (i mean between Navision and GP - i do not know other systems).