Before the price drop for the Nook Simple Touch, it would be the Kindle Touch due to the lower price. I would still say the Kindle Touch is better especially for Amazon Prime subscribers who can borrow one book a month for free (although selection is limited to 5,000 books). Also, borrowing books from the library is fairly easy with the Kindle. Most importantly, the Kindle ecosystem is larger than the Nook and Amazon is even getting into publishing.
I don't think that one is significantly easier than the other to use.
The one advantage of the Nook Simple Touch is that Barnes and Nobles has free classes/workshops at the book stores as well as staff that can help if needed.
The nook also supports ePub, if you're the sort of person who likes to download books from sources outside of the Amazon or B&N ecosystems. Of course, a "not so tech-inclined elderly individual" is not so likely to leave that walled garden, but for other people, it's nice to have the option.