The following are 3 choices from Autozone:
Battery | 2006 BMW X5 AWD 8 Cylinders 4.4L EFI DOHC | AutoZone.com
One reason to consider a battery with greater specs than the original is because of cold weather where cold and cranking amps are more important. The difference in price is not that much and the warranty is usually longer. On the other hand, you haven't had any cold starting problems with your current battery so I would say the specs on your current battery are sufficient.