I find that battery choices can almost be like what type of oil to use.
But for the most part...
any 49/H8 or 95/H9 battery class size will fit/work. And since there's only a few battery manufacturers in the USA that make most all of the battery brands...most of the specs are very similar...with only the battery case's color and brand label separates them from all looking the same.
You can see on the list below that Johnson Controls & Exide manufacture a lot of the popular brands that can be found locally where ever you live in the USA. The list may have changed by now...but for the most part...these 2 mfgs make the majority of the batteries sold here in the states:
The 49/H8 batteries tend to cost the less (respectively)...with the 95/H9 batteries costing the most. One may also want to consider warranty length and the ability for the warranty to be honored. For example, many people select one of the Autozone "Duralast" batteries because they are almost nationwide, and your name goes into their database...so if you were out of town and needed a new battery...your info could be called up regardless of you being from out of town...and if your battery was still under the free replacement or extended pro-rated part of the warranty...it would be honored.
Another example (using Autozone)...they sell the Duralast 49 DL, Duralast Gold H8, and teh Duralast Gold 95R...again, any of these batteries will work in an e53 X5...their specs are almost similar...their warranties are only different by one year...they all vent (some from the top center & some from the side)...and they range from around $100- $135-$190. Interestingly, the
49 DL is the least expensive but has the highest RC (reserve capacity):