Companies always preferred Top down only.
The Reasons are:
1. While doing assembly, it has been save time
2. We can reduce the assembly clashes between the parts.
3. No need to positioning parts in assembly.
4. In companies, they are always use Enovia or Team center, so many members can work in single assembly with different parts&timings. so if they go with Bottomup assy means, they have to assembly all other parts and then they would start to work on their parts. but if they preferred TOP down means, no need to do assmebly of other parts, because it already in positions. so you can start working easily. Top down is user friendly.
5. You can start to create a parts from assembly work bench itself.
6. Any time you can do reassemble easily, because it will come exact position in assy work bench. no need for constrains in assy also.
I hope, my answer will help you little bit to understand which is best.
its depend upon the company you working.
For example: If you Customer providing the boundary's you need to work in top down. Customer didn't provide any boundary constraints you can work in bottom down. But in Tier 1 Company we will work in Bottom down assy for Internal child parts if design freeze.