Before you replace the battery I suggest you have it load tested. The test will be conclusive one way or the other. Keep in mind that BMWs are very sensitive to voltage variations so there should be some urgency replacing the current battery if it test bad and should be considered in what the specs are on the new battery. I wouldn't purchase a battery that doesn't at least meet the original specs. I think the purchase of a battery is not about what is sufficient for normal conditions. It should be about what will still be sufficient in extreme conditions. Also consider the length of the warranty and how long the two types of batteries normally last as part of the pricing decision. The AGM battery could be a better value even at the higher price. Based on the importance of a good battery, it has always served me well replacing original batteries with heavier duty batteries.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas