Lost: Artscroll is currently working on/just finished translating the Midrash Rabbah. I would use theirs. That is not to say that the Sonsino is unreliable, but Artscroll definitely put more work into it (and the footnotes make it worth it).
To respond to earlier posters, it is the exact opposite. The reason Hebrew is so untranslatable is because it is a far less precise language than English. Especially in Chumash (but also in Midrashim), words can have multiple meanings and nuances that any one translation often just misses.