There are two separate ERPs under the Dynamics 365 umbrella, and the names don't make that clear. Setting off with the wrong product is one of the most expensive mistakes discovered mid-project.
01. Different Origins
Business Central descends from Navision and was designed for mid-market. Finance and Operations is the heir to Axapta, written for large-scale, multi-site, multi-country operations. They share no code base.
02. The Scale Line
A rough rule: under 500 users with one main operating centre means Business Central. Thousands of users, multi-plant manufacturing and a complex supply chain mean Finance and Operations.
03. Manufacturing Depth
BC's manufacturing module offers basic MRP, work orders and routing. F&O has advanced capacity planning, multi-site planning and lean manufacturing scenarios. If manufacturing is complex, BC gets tight quickly.
04. Cost Difference
F&O's per-user licence and implementation cost are several times higher, and project duration moves from months to years. A BC project can go live in 3-6 months while F&O is discussed in terms of 12-24 months.
05. The Growth Scenario
The plan of starting with BC and moving to F&O once you grow isn't realistic: there's no upgrade path between the two, and a move means a new ERP project. Factor your five-year growth target in from the start.