Unlikely you would notice the difference. In theory the lighter discs would have a lower intertia which might in turn have some effect on steering and acceleration but in practice the differences are likely to be so small as to be invisible unless you are an F1 driver. The drilled/slotted options can improve braking performance, especially in the wet but usually at the expense of a lot of noise. Sounds great on a track but gets tedious in regular day-to-day driving.

Since we're talking about a 520d here you are not really ever going to be pushing the limits, say compared to an M5. Stock plane discs should do you just fine.

 

On two piece discs only the hub part is lighter so the change in rotational momentum is very small. The effect is more on the unsprung mass like you suggested. It's always beneficial to keep the unsprung mass lower so it's easier for the tyre to follow the surface. Will you notice that difference is another thing. Still the two piece discs are not that expensive compared to one piece units.

https://www.motointegrator.de/artikel/1036016-bremsscheibe-ate-240124-02392-1-stueck

 

On two piece discs only the hub part is lighter so the change in rotational momentum is very small. The effect is more on the unsprung mass like you suggested. It's always beneficial to keep the unsprung mass lower so it's easier for the tyre to follow the surface. Will you notice that difference is another thing. Still the two piece discs are not that expensive compared to one piece units.

https://www.motointegrator.de/artikel/1036016-bremsscheibe-ate-240124-02392-1-stueck

Thanks for the link to that german webshop, seems like really good prices. Do you have any experience bying from them?

 

also the 2 piece do not rust on the bell,my ebc ones look great,like an anthracite bell

 

That is also a bonus. No need to paint the hub piece to prevent it rusting.