I am just buying a BMW AGM this afternoon for my 4.4i.

I know it costs about double than a Sealed Lead Acid but I'll give it a chance.
I do NOT drive my X5 a lot (or any of my vehiccles, for that matter), and when I do its been short trips, so no time to slightly charge/maintain the battery when I do. Hence why the X5, when parked in its Car Home) is almost 80% of the time connected to my C-Tek charger, which is suited for a Group 49 (H8) battery values.
I've used this batt tender for 18 years now on a few different vehicles and even have a larger capacity one for my e31 (has 2 Group 93 batts) and another for the R55.
I have a new (3 years ago, which is about 8k kms ago) alternator in the X5 and this battery is also about 3 years old.
I had no parasitic drain until I installed about 5 months ago, a Wolfbox rearview mirror back-up camera system and then I noticed my batt had deep discharged about 3 times until I figured it out... but the entire 35days I was not using the X5, the mirror wasn't connected to the battery and the X5/battery was connected to the maintainer on maintenance/storage mode. I have been told numerous times now that vehicle batteries are only going to last cars for abouit 2-3 years now before anew one is required.... I hate that its true, BUT at leaset its not a huge expense.

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I swear, my cars are like a girlfriend.
Sometimes its a rough ride, sometimes its smooth motorin'.
Sometimes she doesnt like how i treat her and sometimes i dont like how she behaves.
BUT at the end of the day, she loves it when I am inside her.

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'91 850i
'05 X5 4.4i
'09 Clubman S