It's easy to get residence status (IIRC basically requires a ~$60k investment in a business you are starting) and extremely easy to start a business. You get access to the EU labor/trade markets in a low COL city which already has a decent sized tech and entrepreneurship presence. The government is actively courting you and in general seems very easy to interact with and move forward with - reducing the PM/lawyer overhead of operating a medium sized business in other countries. And if you stay there long enough it's not overly complicated to get EU permanent resident status.
Overall, Tallinn beats SV in almost every way except for two - developer talent/concentration (though it's still pretty good) and local capital.