It's a luck of a draw, really!
S63 engines on X5M are more reliable over all, that's the case if maintenance done properly but you won't really know it for sure even if you have all the records.
You mentioned reliable, low costs etc., all these are not the words to describe owning BMW especially M models!
I never owned Infiniti but I owned several Japanese cars such as Maxima etc., most of them were more "reliable" and less expensive to own!
Now I read about lots of issues with 50i but then again some have those issues and some don't.
Me personally coming from X5 35i in to X5M few years ago was a shocker
Good luck in your research!