I have a 4.4i and if the price for the 4.4 is cheaper than the 3.0, I would deffo consider the V8.
You get virtually the same MPG on the V8 along with more power, more torque and a lovely engine note.
Just make sure you check for any oil leaks, you should look for one with a totally dry engine underneath. If you can get hold of a code reader also, I would recommend checking the live data readings to see where the fuel trims are sitting. The long term trims should be no more than + or - 5. Anything higher will give you horrible economy and possibly expensive problems to diagnose and fix down the line.
After owning an X5 for the last 12 months, I wish I knew all the things I know now after a huge amount of research and work, before I bought one. I would have still bought one but would have known what to look for..... The list is massive!