SSAS Tabular Vs SSAS Multi Dimensional

Well that question totally depends on what features you need to build the cube. There is still some very nice features in Multidimensional that isn't available in Tabular. But if you can build the cube you need in Tabular then I would go that way. Power BI's own models are build on the same engine as Tabular models hence there is a better "understanding" between Power BI and Tabular than Power BI and Multidimensional.

A few weeks ago I attended a session with Chris Webb where he talked about this question and he listed several things that isn't supported in Power BI when using direct query to Multidimensional, but also stated (and I agree with this) if you already have a Multidimensional cube you should only switch to Tabular if there is things you need to do in Power BI that is not supported for Multidimensional. However if you want to start over and build a new cube I would go with Tabular unless there is features that you need that is only supported by Multidimensional.