Hi
I have experiences with both branches, I volunteered my time at both branches some time last year in order to finish my course in child care. In terms of teachers, you will not go wrong with both branches. All the teachers in both are highly qualified, and its easy to see that they love their jobs. Teaching staff at both have been there for years and years. Although the safa branch sees a higher turnover of staff as it is a much bigger branch.
When it comes to curriculum, safa branch is more established in terms of the EYFS curriculum that they follow. The wasl branch when I was there were still starting to get grips with EYFS. Not sure how far they have gotten with it by now.
Safa branch offers more activities eg: swimming, they have a pool on the site, they have a really impressive water play area, sand area and quiet a few out door areas for the children to enjoy(weather permitting) They also have a big soft play indoor area with different sections for the kids to enjoy. The younger children also have their own play area. They also have music once a week, arabic lessons and ball skills. Can't remember what else(sorry)
I think it all boils down to what you want. Go have a look at both and make up your mind from there. Either way you can't really go wrong.
Good luck and let me know if you want any more info