Unless yr 80 years old grandma just retired as a wild life photographer or just came back from climbing aconcagua, I say she would appreciate one of those bug flip phones with the large keypad and screen.
Great news I bought my Grandma the White/Gold iPhone 5s and she simply loves it!
All the Grandmas at the Bingo hall love it as well, sounds like they are asking for presents from their Grandchildren, top on the wish list for Grandmas is the Gold/White iPhone 5s.
Oh and for all the haters, Grandma sends her love and says this is a lovely upgrade from her old iPhone!
All though it appears that Silver/White is slightly ahead of Gold/White for Grandmas. Perhaps both are great options for Grandmas and the Space Gray/Black would be great for Grandpas!
My grandma couldn't use a mobile, so none of them. Typically though, I don't think they would make full use of the functions on the 5s. I acknowledge though that iOS is easy to use as far as mobiles go, so maybe a 4S?
I think silver is the safest bet, but definitely not the space grey in case they don't like the black bezel. I think there is more not to like about a black bezel than a white one.