I have used both Datastage and Informatica... In my opinion, DataStage is way more powerful and scalable than Informatica. Informatica has more developer-friendly features, but when it comes to scalabality in performance, it is much inferior as compared to datastage.

Here are a few areas where Informatica is inferior -

1. Partitioning - Datastage PX provides many more robust partitioning options than informatica. You can also re-partition the data whichever way you want.

2. Parallelism - Informatica does not support full pipeline parallelism (although it claims).

3. File Lookup - Informatica supports flat file lookup, but the caching is horrible. DataStage supports hash files, lookup filesets, datasets for much more efficient lookup.

4. Merge/Funnel - Datastage has a very rich functionality of merging or funnelling the streams. In Informatica the only way is to do a Union, which by the way is always a Union-all.