There have been good reviews on both and both are reputable. The ride will be fine and you'll get to see some of the beautiful country-side. They do have air conditioned taxis. The last time we did Montego Bay, we took a tour up to Rose Hall, then they stopped at a Rum factory, then shopping. The time before we went to the marine park, but that was years ago. Rose Hall was about 8 years ago and it was a good tour.
I think Beaches does Sand Bay and Negril. I think Sandals does Negril and their own private beach. Either one sounds great to me.