What's the right price? I suspect they're both the same but I've only bought them for a GSA which I think will also fit a GS.
I reckon the BMW Gel Exide batteries are okay, last for years, fit every time, and for the sake of a tenner on a cheaper one probabley will not matter over its lifecycle.
Don't bother with the NASA batteries unless you can connect it correctly and have some money to waste.