Hi Brian, I have personally been to Sandals Montego Bay a few years ago and really loved the resort. The beach was beautiful and the service was top notch.
Sandals Royal Caribbean does have the off-shore private island, something Montego Bay does not.
For an anniversary, all you would have to do is let your travel agent know you are celebrating and they would note the record. Usually the hotel will do something extra for you in honor of your special occasion!
As far as amenities (pools, restaurants, and bars) they are very similar in how many they offer. You also have the dine around plan with the share privileges between the resorts.
I think either resort would be a great choice. In terms of price, Sandals Royal Caribbean is slightly less expensive, however, not enough to make a significant difference.
Hope this helps your decision! Here is a link with some further detailed information - http://www.goawaytravel.com/html/sandals.html