I've been to both properties twice in the past year. With one exception, you won't go wrong at either one. The best one for you depends on a couple of things.
KB
Hotel is located within a gated community on Key Biscane. It is about a 20 minute drive to south beach and other happening places. If you're looking for clubs and mini-skirts, this is not the place for you.
The property is directly on the beach. The beach is, IMHO, one of the best beaches in Florida. The sand is soft, and the beach is very wide. I find the beach to be largely unused as well. It is very swimmable. The property has two pools, one of which has a beach-style 0-foot entrance and a waterfall...the other is now in a private area and limited to adults only. This is a really great setup. The club level has a small club hut (I recall it had a fancier name, but I don't remember exactly what it was) that had non-alc drinks and other goodies for club floor guests between the pools and the beach.
The club lounge is ok, and I'd put it at a standard ritz club level. Nothing over the top, but adequately sized. Dining is also standard ritz, nothing superlative.
The property itself is fairly large, with probably 300 rooms or so. They have a few residential suites, with kitchens, if that suits your need.
The garden level rooms, which I've never stayed in, supposedly have parking lot and service alley views.
The big drawback, however, is this is a conference hotel. I stayed here for a week one time in one of the residential 2br suites and was forced to listen to the salesman of the year award for the national association of all state salesmen (or some such thing) as well as the southeast chapter of the national restaurant association dinners in my room. The lawn of the property - which is great when no events were going on - is between the two pools and is the spot they use for outdoor functions for conferences. If there is an event going on while you are there, it will definitely impact your stay. I've been there when no events were going on, and it was great. You should speak with the hotel about your dates to understand what events are planned.
The hotel is 15 or so years old. In decent shape. Standard ritz decor, quality, etc. Service consistently decent.
FLL
This is an interesting property. It was built to be flagged as a St regis. A year or two after it opened, it was re-flagged as an RC. It opened right into the Gfc, and was a much more luxurious property than had been in FLL ever before, and with a brand that is still largely in start up mode. Bad formula...so now a new flag. The nice part is the building is quite new, and the property is in great shape.
The RC, like almost all hotels in FLL (there are exceptions) is across the street from the beach. The pool is on the [8]th floor, and overlooks the beach - but is high above it. There is a private terrace on the 7th floor that you can reserve cabanas on, etc. The pool is only open till about 7pm, but gets substantially blocked from the sun (the tower portion of the building is directly west of the pool) by mid afternoon.
There is a bridge across a1a (the road on the beach) from the pool deck to the beach, so you can get to the beach without dealing with traffic.
Ft lauderdale beach is also a great beach - very swimmable, soft sand, very wide. But it will be much more crowded than the kb property - and have a lower, but not too low, socio-economic feel to it.
I found the service at this hotel to be very good, half a notch better than kb. They have some very nice 1br suites. Once, when I booked one of these during Christmas week, I was upgraded to the ritz carlton suite which was very nice. My other stay also had an upgrade, though not as generously. (but I was still very happy)
The club is one of the best club rooms in the united states within the ritz portfolio. Very complete breakfast, and much more generous food offerings throughout the day. The club is new, and only opened last year - as when it was built as a St r, there wasn't a club concept. I've not been to the new la ritz club, but of the 15 or so properties I've been to in the US, this one is the best. I do think, however, that the club may close during certain summer months. I've only been in the winter...but call to ask before you book.
Dining was decent at the property as well.
The location in lauderdale is interesting. This is the most upmarket hotel in the area. There are other nice places coming up, but this was groundbreaking in terms if the level if luxury for the area. A wide variety of surprisingly good dining options are nearby on las olas blvd, as well as decent night life. This is a 10 minute cab ride away. There is other activity more nearby, but will generally be slightly lower end.
One nearby restaurant to draw your attention to is Steak 954, which is walking distance from the RC FLL. (I am a NY food snob....in the US, there is NY food, and what everyone else does to try to catch up.) steak 954 nearly does it! Some of the best dining experiences I've had outside of NY (within the us) have been here. It is at the w hotel, also right on the beach. Indoor and outdoor options. If you like steak, I highly recommend this place.
I have a review of the w which talks about this, which I'll post a link to after this post.
The FLL property also has poolside rooms - hotel rooms on the 8th floor which open directly to the pool. If you are travelling en famille, you might consider these. FLL is certainly a more lively location than kb - but not too much so.
Hopefully this gives you some flavor. Probably the most palm beach-ish is the kb property, and the beach there is wonderful. Make sure there is no conference underway, however. Barring a conference, you won't go wrong at either property.
Make sure you book either through Amer FHR or a ritz preferred agent to get the extra amenities. (Amex offers upgrade if available, free breakfast for two, guaranteed 4pm late checkout and some other credit, the details of which I forget). The ritz preferred agent program is similar, though the late checkout isn't guaranteed (though likely to be honored).
Let us know how you go.