If I am doing a really complex assembly (350-400 components), which approach of the two should I utilize - top-down or bottom-up ? Will really appreciate if you support your answer with little reasoning. Thanks !

Inner components out, less turning on and off visibility of certain components. But from the bottom up would be my suggestion I guess, it makes it easier starting point if the bottom is centered at the origin for making exploded views and rendering usually. It depends on the assembly.

You can have a mixture of both. You can create the top level structure using top-down methodology and use the bottom-up approach to work on lower level assemblies. For an assembly this size, I would use a top-down structure.

For a design project where considerable information exists e.g. from the experience of a similar project, top-down approach has a clear advantage.

If very little or no information exists about a new product, it becomes meaningless to start with top-down approach. In this case, bottom-up approach makes more sense.

There may be situations when both techniques are used together in a same project. For example a project may be initiated with bottom-up approach. As the design progresses and more information and constraints become known, the design project can be handled more efficiently by adopting top-down design approach. Similarly, in a top-down design project, to place the standard components like screws, bolts, nuts, bearing or other components that are used across multiple projects, bottom-up techniques are preferred.

You can have a look at the following blog post for detail description.

http://caeuniversity.com/2015/03/16/bottom-up-and-top-down-approach/