If it were me I'd forget about the performance differences, you'll get used to whatever you drive. Think more about whether you want a high or low driving position and a great feeling of space or snug fit.
I came to the X3 after two 330is and a 335i. The 3s were great but I struggle now to adapt to how low they are (they are lower than most cars), they also feel cramped.
Just go with your heart.
F25 3.0D | M Sport | Black Sapphire | Oyster | Fineline Wave Wood | Heated Front Seats | Panoramic Roof | Matt Aluminium Roof Rails & Window Trim