Hi Malcolm,
Just remember to double check before you hit "Add to Basket" that it is an
AF-S lens, I am not convinced that the "G" designation is synonymous with "AF-S"; and the latter will be necessary for it to Auto Focus on your dearly beloved's D5100.
I'd strongly recommend doing a shoot with the 17-70mm and see which focal lengths you use or need, it may suggest longer is the way to go, but remember the versatility of a zoom is hard to beat.
If the apertures you're going to use will be in the range the 17-70 has already, why duplicate that range?
I am not sure extra sharpness (ofa a prime) is necessarily something that's really needed for portraiture, probably just end up smoothing it out again in PP to give a better complexion
Cheers,