I don't really like to use the word "compare" - each person brings their own dimension to a role and comparisons are sort of meaningless.
However, having seen both Gabby and Sydney twice, I didn't feel the show suffered or gained by having one girl over the other. They are both immensely talented performers.
I would have liked to see Sydney do the show when she started at the Public Theater and she was younger...she was pretty tall and mature by the time she got to Broadway. Gabby though actually older by a few months is much smaller and gives a younger feel to the role, which works well IMO.