In fact choosing between these two techniques depends on your design intent. However, there are some considerations regarding the top-down design:
- Whenever you create a part or feature using top-down techniques, external references are created to the geometry you referenced.
- In some cases, assemblies with large numbers of in-context features (which form the basis of top-down design) can take longer to rebuild than the same assembly without them.
- When creating in-context features, it is important to not create mating conflicts because they can cause long rebuild times and unexpected geometry behavior. You can generally avoid these conflicts by not creating mates to geometry created by in-context features.
Reference: 2013 SOLIDWORKS Help - Top-Down Design
Aside from that there are other techniques to speed up your assembly performance, no matter how large and complex it is, such as Large Design Review mode (LDR); Speed Pack, etc.