Because both HS7 and HS8 offer a flat and accurate sound, both are great for mixing tracks. However, the HS8 will definitely work better in a professional recording studio due to its power and bass response. The final mix will also translate well to other audio systems, such as your car speakers, cellphone, headphones, etc., and give you honest feedback on your music on all systems.
Keep in mind that the HS8 is a very straightforward monitor, meaning that if your mix is terrible, you’re going to hear all those audio mistakes and imperfections very clearly and obnoxiously, so make sure you mix well before giving it the final test.