Just finished a week at Pearl, first time there. Had been to RM three times. There are pros and cons to almost any two things being compared and the same applies here.
Without making the too long and detailed, here are our thoughts:
Rooms:
Pearl rooms are nicer but not significantly so. Really, how much time is spent in room? Both are well maintained, clean.
Pool:
Pearl. There is a bar in the pool at Pearl, not so at RM. (Admin Update: as of Oct 2014 Desire RM now has a swim up bar in the pool) The layout of the Pearl pool is also better as it extends almost the length of the inner property. One end is quieter, if desired while the other end is more active and social.
Beach:
RM. Water is calmer. Many more people in the water. Very few enter the water at Pearl. Also, a bit too many people from outside of resorts walking by Pearl beach. I don't mind the adults but there were a few too many kids (in the 10-15 year-old range) which I find uncomfortable. I just don't think 12 year old girls should be looking at men's wagging tails. The little kids (6ish and under) aren't really an issue as they are too young to understand or care about the nudity.
Hot tub/Jacuzzi:
Pearl, hands down. Better location, bigger, nicer. No stairs to climb. Close to pool but still secluded. Few would argue this.
Food:
Frankly, I don't understand how anyone rates Pearl's food over RM's. Of course, everyone is entitled to their opinions but for us, it was not contest. The food at Pearl is below average. RM is average to slighly above. There is also more choices at RM as Pearl only has two restaurants. Also, the format at RM is much better for with the afternoon restaurant being outdoors. At RM, there is a grill that offers fresh fish, burgers, pizza, etc. all day long and you can eat outside. At Pearl, it is closed in lunch area and a bit too formal for casual dining in bathing suits. As for Pearl's main dining room, the room itself is nice but the food is subpar. The servings are extremely small (ordered gnocchi one night as main course - serving was four gnocchi's, each the size of a chocolate covered almond). Everyone at table laughed. There are no dinner buffets at Pearl. At RM, the dinner buffet is offered as an alternative to the restaurants, which is nice when you don't like the daily cuisine of the restaurants and would rather just pick and choose your own thing. Additionally, part of the fun at RM is the late nite pizza being served up at the bar. While the same is offered at Pearl, it is from a makeshift pizza stand and it pales in comparison to RM's.
Staff:
Great at both. Super friendly, thoughtful and nice. No complaints except to say that Sylvia at RM does not have an equal serving drinks around Pearl pool.
Atmosphere:
Met some great people at both. However, there is a better 'buzz' and a higher level of participation in theme nights. We found that Pearl was a bit more like a summer camp where everyone moved in packs to whatever the current event was. 10 am at pool/beach, 4 o'clock hot tub, 8-10 dinner, 10:30 disco, 11:30 hot tub. There wasn't much variance from that. At RM, it just seems more diverse. Also, the social area just behind the lobby at RM is much better than any at Pearl. We always enjoyed hanging around the bar at RM if we weren't in disco or elsewhere. It was fun, lively and social. Same can't be said for Pearl. They lack that 'spot' .
As for the 'party', I do believe that much of it is decided by the people there at any given time. It is said that Pearl is tamer and perhaps that is true but either property offers as much 'partying' and 'lifestyling' as one might desire.
Keep in mind, we are not wild partiers. We love the atmosphere and ambiance of Desire. For whatever you're looking for, it's available at either property. But for us, we'd go RM before Pearl.