While I was looking for an X5 and asking around from 4-5 different local independent shops specializing in BMW, (and my service advisor at BMW dealership when I was there for my wife's 328i maintenance), EVERY ONE OF THEM told me to go with the 3.0 engine for better reliability and less overall issues. That is why I went for the 3.0 engine.

Now I am sure the 4.8 engine will be a blast to drive (haven't driven one yet... didn't want to spoil the fun for myself knowing I wasn't going to buy one anyway), and from feedback on this forum, it seems like anyone who owns it loves the drive... but if reliability and lower cost of repairs is important to you, you may want to consider the 3.0.