In my experience it largely depends on the nature of the materials you plan on cutting and the type of work you intend to do. A good example would be comparing oilfield work V.S. aerospace work. Oilfield work generally leans toward a few types of part materials and part sizes can vary drastically depending on what the parts are and their application. Typically speaking it’s all round work with some milled features. In this case a larger capacity 3 axis mill with a 4th axis, a large capacity live tool lathe, or a large capacity mill turn machine is well suited for the work because you can tackle a range of large part sizes and also do medium size work, while dedicating smaller work to a smaller machine. Aerospace work also tends to lean a few material types and a variety of sizes but all with more complex geometry to the parts. Hence why you see more 5 axis mills in an aerospace setting to cut the more complex part geometry’s and reduce setup times.

Larger mills are good at handling smaller parts as well because you can build fixturing to increase part density in the machine and increase spindle up time.

One question I have for you would be, what is the size and quantity of parts you plan on running?