My parents have bought two X5's so far, and both of them are thoroughly disappointing compared to BMW cars imo. JCL pinned it, BMW or not, it can't deny it's weight and center of gravity. In your case, not only will the X5 handle much much worse than the 335, it's also going to completely lack balls compared to the 335's engine despite being a 4.8.

Keep the 335 for yourself, get her the 3.0i like she wants.