Prefer personal blog, if you are worried about the reach, post on both. I think Medium even still allows you to mark a post as a copy of your original, so it sets a canonical url reference on it.
I would not trust Medium, especially long-term, and it already has quite a bad reputation in some circles (i.e. I personally tend to avoid Medium links if they don't come well-recommended). If they change in a direction you don't like, you've tied your writing to them - your own domain is a lot more flexible and safe in that regard. On the other hand, people do report larger reach on Medium if they aren't already established. Crossposting gives you access to that, while also maintaining your own place on the web.
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Prefer personal blog, if you are worried about the reach, post on both. I think Medium even still allows you to mark a post as a copy of your original, so it sets a canonical url reference on it.
I would not trust Medium, especially long-term, and it already has quite a bad reputation in some circles (i.e. I personally tend to avoid Medium links if they don't come well-recommended). If they change in a direction you don't like, you've tied your writing to them - your own domain is a lot more flexible and safe in that regard. On the other hand, people do report larger reach on Medium if they aren't already established. Crossposting gives you access to that, while also maintaining your own place on the web.