Both anime are based on Adachi Mitsuru's works. This alone should tell everything you need to know. Same author, theme and similar characters but in a different set-up that makes it interesting to watch Mix as a new series and at the same time feel nostalgia for Cross Game and all of his previous works. If you have seen Cross Game and loved it, you will probably like this series also.
While both being Adachi Mitsuru's works they also share the similarity of potential love interests living under the same roof. Hiatari Ryoukou is quite old but totally worth watching since the characters are great. Also both have baseball, of course.