Thanks for elaborating. I make it a principle never to lend my car to people I don't trust to treat it right. Now that I think about it, the only person who has driven it whose name is not on the title is my dad. Even though it's a far cry from the Series 3 and Defenders he used to drive during his years in southern Africa, he managed just fine.
If I may attempt a summary: AGM isn't a bad option, but it has limitations that a flooded battery doesn't. Assuming the warranty discount applies, replace the Interstate with like-for-like and spend the savings on the next trip (be that to the mountains or to the shop).