Wear the dress that you feel best in. Wear the one that makes you feel beautiful, like a queen, like the most wonderful grandmother of the bride there is in the whole wide world.
You're the grandmother. No one is expecting you to look like the bride. I recall fussing over finding the best ever mother-of-bride garb and soon found out, no one cared. It was not my wedding! For one child's wedding I was champagne and lace, another I was royal blue elegance the third I was ...goodness, I can't even remember. All I know is that I felt good, looked smashing and had a wonderful time visiting with friends and new family.
Wear the dress that is all YOU!
If you really want a good answer ask the bride. Give her two choices. No more than two because she has been making too many decissions already, both of which sould be acceptable to you and get her two cents. If any one criticizes, you can say the bride helped you pick it out, she gave her approval or "in consultation with the bride we thought this was the perfect choice." Enjoy the wedding.