If I were you, I'd pick something that I liked, because if your grandma is anything like mine was, she's going to use the yarn to make something for you!
If she's used to acrylic, better get something washable - maybe superwash merino. Worsted weight or bulky works up quickly, and may be easier on arthritic hands than something fine, too.
Consider giving her a homemade "gift card" for one of the projects you started for her, and then spend an afternoon with her making it and getting her help with it. She'd probably love the time with you more than anything else.