I have gone through every line of the entire Soncino translation. I did this when I was writing my own heoros to Midrash Rabbah, which was published in my book, ????? ??????. I can tell you it is an outstanding translation, and 100% perfectly reliable, even to the “frummest” reader. It is all based on the standard meforshim of Yefeh Toar, Etz Yosef, Maharazu, etc, all of whom are quoted by name in the footnotes. Even auxiliary meforshim like the Rashash are cited.
The Artscroll translation is also very good. I used them too, although parts of Bamidbar Rabbah was not yet available when I was writing my sefer. Obviously Artscroll has a more yeshivish approach. They spend a lot of pages explaining basic points that anyone with an elementary Jewish education already understands (and Soncino takes for granted. Like explaining what “terumah” is, things of that nature.) They also have a big section citing the Ballei Mussar. Anyone with any sophistication whatsoever understands that neither approach is better, they are just different.
Interestingly, both the Soncino and the Artscroll cite the academic Theodore-Albeck edition on Beraishis Rabbah. Artscroll also engages in classic Artscroll censorship (on an Eitz Yosef) as I proved in a post on Seforim Blog, where Dr. Marc Shapiro cited me on this point. You can google it.
Bekitzur, as one of the few (only?) people I know who have actually gone through both translations, I can confidently say they are both excellent pieces of scholarship. Either one is well worth using when you study Midrash Rabbah.
David Farkas
Cleveland