We stayed at the Grand in June and were upgraded to a multi floor, multi room suite with two bathrooms and two balconies overlooking the Arno. Our girls had a nice room too, but not a suite. Nonetheless, if the girl's room had been ours, we would have been equally pleased. It is beautiful hotel.
(We also stayed at the Excelsior in Rome and the Europa Regina in Venice)
Of the three, I can't say enough wonderful things about the Grand and it's staff. It was definitely our favorite of the three hotels. And the staff all genuinely seemed to want to make our stay wonderful (which they did).
Efficient turn down service with new towels in the evening, wonderful bedding, very patient and friendly and supportive concierge staff unlike Rome where they were polite and efficient but acted like you were interrupting them in the middle of their doing something else.
We did use the concierge in Florence to get us tickets to the Ufizzi and to the Academy and I think the service charge was maybe 3 euros per ticket, well worth the convenience.
From a location point of view, the two face each other in a open square so location is a moot point.
I don't see how you could go wrong with either. If you can get a room for 300 euros, it is a steal in either!