Personally, I like 6-cylinder BMWs. They are smoother - they are inherently smoother it is just not possible to make a 4-cylinder engine as smooth as it is possible to make a 6-cylinder. So if it is in your price range, I like the 528 (caveat: have not driven either a 528 or a 520 or any F10 5-series).
I have the 3-litre diesel. Overall, coming from a 6-cylinder gasoline engine before, the diesel makes more financial sense, but it is less fun and still sounds a bit like a truck.
BTW - I've taken two driving holidays in Taiwan, great country
