4.8 give mores power and fun, with poorer fuel economy and reliability.

3.0 has better fuel economy and reliability, but less power and less fun.

BMW I-6 engines are sweeter engines than BMW V-8's, in my opinion, but the X5 is a very heavy vehicle.

Your choice.

Key thing with any used BMW is low mileage and previous good care. I personally would not buy a used X5 with more than 40-50K on it, I'd pay more for a low mileage one or save my pennies to buy a new one with 4 years of warranty.