I don't like either, but if I had to choose, I'd pick
Roberta. I just like its sound slightly more than I do the sound of
Robyn. I think that it would be better also, because it would honor both your mother and grandfather. If I had to pick between
Robbie or
Bertie as a nickname, I'd definitely pick
Robbie, because I don't like the "bert" sound at all. It's why I don't like
Robert. Normally I'd prefer the
Robin spelling, but if you were to go with
Robin /
Robyn to honor your mother, I think you should use the spelling she does.