My choice: two separate attributes. Build dozens of apps storing location, and I have always been using two separate attributes. Number of decimals needs to be 8 or more. Take care that upon adding the attributes to a page, it only will show 2 decimals. You have to manually change that to 8 every time you add the attribute to the page.
Having said that, calendar exports to ics concatenates lat&long into one field GEO .
As far as I know there is no ‘Object’ to save both in one place. So I would recommend to store both in a seperate decimal attribute.
Of course you could create a string and store both values in there, but that will be more difficult to work with, so I wouldn't recommend doing that.
Thanks a lot guys for sharing your expertise here, I can see that you are very familiar here!