We've been to both resorts and just returned last night from 8 nights at Desire (4 at RM and 4 at Pearl). We had Friday night at RM as our last night and Saturday night at Pearl as our first night so we could experience a weekend night at each. We have been to Pearl a number of times before for a week at a time. It is true that the vibe is set by the crowd that is there at the time. We saw more action going on at Pearl on Saturday than we did the whole 4 days we were at RM. Ages varied at both as well as bodies style so don't plan on a "hard body" setting at either.
Each person you ask will have their preferences and we find an even mix between Pearl and RM preferences. If you can swing it, we would suggest 4 days at each. Any shorter than that and you really don't get into the swing of things (no pun intended). If you can't spend at least 4 days at each, then pick the one and stay there.
Pearl: Nicer rooms (88 total), more open space on the grounds, huge pool and giant jacuzzi, we think the disco club is better in size/layout,
RM: Maybe better beach, 114 rooms but varying degrees of style from spartan to luxurious, small pool and jacuzzi, disco and jacuzzi up a flight of stairs (not a good combo with sexy heels and drinking). Habatchi grill is very good.
Staff at both are top notch.
You can't go wrong with either.
Remember - even a bad day at Desire beats a great day anywhere else every time! :)