If you're talkin the new 07 X5, there is no 4.4i. There's the 3.0si and the 4.8i and basically it depends on the power and level of equipment you need. If you start finding yourself piling on a lot of options like premium package and content that's otherwise standard on the 4.8i, then get the V8. If you're just looking for something like sport package and not much else, then go with the 3.0si.