Omme, we stayed at the Ambassador two summers back and enjoyed the hotel; it's a lovely hotel situated in an excellent location at Platys Gialos beach. We had tried to stay at the Grand during both of our visits to Mykonos; however, the hotel was fully booked each time. I'm sure you would enjoy the Ambassador, but I think you would enjoy the Grand far, far more.
I have not yet stayed at the Grand, but have read a great deal about it both in TripAdvisor reviews and in postings on these message boards, as well as on other online sites. I also have seen dozens of photographs of the hotel property, including a few taken just last month by a TripAdvisor member who stayed there for his honeymoon and absolutely loved it. (Indeed, in postings on this message board, he highly recommended the Grand in response to a question asking which Mykonos hotel would be ideal for a honeymoon.)
I'd recommend choosing the Grand instead of the Ambassador because it clearly appears to be a superior hotel in a better location. The swimming pool at the Grand looked spectacular in the photos I've seen, as did the sunset views from the hotel rooms. And based on what I've read, the service at the Grand is exceptional.
Since the room prices you've been quoted for both hotels are similar, I think you would get much more value for your money and much more satisfaction if you choose the Grand. I'm hoping to stay there during my next trip to Mykonos (provided, of course, that it isn't fully booked once again!).