"our first priority is nicer beach"
Both are very good beaches. Of the two I might judge the beach a the RIU Palace Tropical Bay slightly better but the difference is very small.
"since these are AI's which one has good food, cleaner facility, pools."
All a matter of opinion.
"any included features that you know would make one better than the other?"
You may rate the nude beach at the Grand Lido a plus. Some, on the other hand, would call it a big negative.
Two other points to keep in mind. First, the Grand Lido is adults only while the RIU accepts children. Second, the Grand Lido is under new ownership and will be remodeling the hotel. You should call the Grand Lido to find out there schedule for remodeling as this may/may not impact your vacation.