Which one should I take??? 523i or 528i

Yesterday I was near the BMW dealer and I saw a 523i and 528i. And since both engines are indeed R6 engines, I could get rid of the so unloved and uneconomical drunker V6 without big losses even if I change the brand then. They offered me to drive both times sample. My C240 would take at least to the Schwacke purchase value in payment possibly also something on it.(After review) Both vehicles are switchcars and BJ. 2000 under 50,000km Lau fleistungs. Does someone know each other with both engines ??? And what weaknesses should one pay attention to???, if BMW has any at all. *twinker* gruss oppa