I have written several lengthy reviews on both of these spots (Sun and Royal) You really can't go wrong. These are two of our favorite spots and have been to both 4 times. There are differences but both great spots.
Royal: great location, lovely beach, amazing oceanview rooms, great food (the steaks weren't great the last two times we were there), close to clubs, Margarittaville right across the street, La Isla mall close by, big evening shows in the lobby each night, a beach party on the beach weather permitting, smoking is allowed in the lobby area/bar (bad for us good for others),
Sun Palace: Best service, $1500 per room for tours, spa, dinner on the beach etc...11% tax but still if you use it all $165 for tons of stuff, sleek rooms (not as fancy as the royal...just a different look), close to the airport...further from the clubs, beach is even better than at the Royal...softer sand, amazing padded loungers on the beach vs normal ones at the Royal, umbrellas for everyone....extremely limited shade at the Royal, small lobby bar but lots of fun stuff going on...karaoke, Anything Can Happen night, DJ..dancing,
Couples night, Michael Jackson show in ballroom, ....we have had enough big shows so we like the entertainment here better, pools are smaller but all infinity, food is excellent....
Get oceanview at either....unless you want double beds don't get the basic room at the Royal...don't get sucked into timeshare at either
-bus ride $1 from either hotel...about 10-15 to clubs/shopping from Sun. We always thought it was far but it isn't
-Royal looks more grand
-lots of activities at both....water volleyball, water aerobics, beach volleyball etc. It seemed that more was going on at Sun re activities but both are great
-Lunch time you can eat outside at Sun Palace at the grill, you can even order food right to your chair or go inside to buffet or room service
-Lunch at the Royal you cannot eat at your chair. There is a sit down restaurant (Pelicanos) that is partly open air overlooking the water....excellent. Some people are upset that you can't bring food to your chair there so wanted you to know that.
-Drinks great at both...fancier plastic glasses at the Royal
-you can order bottles of wine and champagne to your room at Sun Palace...NOT at the Royal
-great mini bar at both and full bottles of booze in room at both
-At Royal you can use Gran caribe hotel right next door, At Sun you can go to Beach Palace
Any questions feel free to ask