Rollups are great for getting summary aggregate data between a few report suites. What rollups are not good at is doing in depth analysis like breaking down variables and using classifications and stuff like that. They just are not very robust and should be used for Summary data only. Pros: it's free Cons: not super deep information.
Global report suites on the other hand are very useful. They can provide holistic views of traffic and how it moves across your different domains. Deduplicated visit/visitor totals to all your domains. Segmentation to get to actual smaller site information. Pros: super deep data analysis Cons: more server calls.
I hope that helps set expectations between the differences of each in a quick statement.