The PPR is a corporate-run beach resort like you might see in Hawaii, just on a smaller scale. Rooms are large and comfortable, especially the automated Japanese toilets. There's an air-conditioned fine dining restaurant as well as a casual outdoors one with menu and buffet options, food is excellent at both. There's a pool bar and a lobby bar. A swimming pool and a gift shop. Wifi. Service is friendly and efficient. Except for the difference in the quality of the diving and the wood carvings available in the gift shop, you really might not notice that you're in Palau and not Kauai. It's a very comfortable place to stay before or after a liveaboard. It's also a very comfortable place to stay for day-boat diving, though it was too rough for the Neco Marine dive boats to pick us up at the PPR's dock, so we had to get a ride there. Adding a possible drive to a long boat ride makes for a long day of diving to get only 2-3 dives in per day, so I definitely prefer a liveaboard in Palau.
The Carolines resort is very "authentic" looking, but I was sweating profusely after hiking around the resort when we toured it - it's built on a steep hill and if you're in one of the bungalows further up, expect a hike up and down that's nearly as steep as the one you have to negotiate to get to Jellyfish Lake. It's probably quiet and secluded-feeling at night, but I'd rather stroll a flat paved PPR path after a night of dining and drinking than have to climb a mountain
If you're there mainly for the diving and don't care whether you're sleeping in a hand-carved wood bungalow as long as the bed is comfortable and the A/C works, I think the PPR is a better option. If you want more of a cultural experience, are planning on touring on land (don't miss Peleliu), visiting the jail for storyboards, etc., and are in decent physical shape, the Carolines might be a better fit.
As for dive ops, I definitely got the feeling that Splash catered more to Japanese clientele, probably because I had to ask three different people before finding one who spoke enough English to rent me a tank. It may be more convenient, but unless you're fluent in Japanese, you'll probably have a more enriching experience with one of the dive ops that cater more to English-speaking divers. Compared to the rest of what Palau has to offer, the house reef is quite dead, so I wouldn't stay at the PPR for quality snorkeling. It sounds like snorkeling and shore diving might be better around Sam's dock from what I've heard, but I do like the PPR's pool.
The PPR is certainly more "noisy" than the Carolines, given that there are more people, a swimming pool, and bars. Still, given the largely Japanese clientele who aren't usually prone to being "loud", it's hardly the noise that you might find at an all-inclusive in the Caribbean, say. The only "loud" activity we noticed on my last trip was one day at lunch when there was a party of three American military from Guam on R&R, accompanied by three women that probably weren't their girlfriends or wives. They were drinking and getting a bit rambunctious with their lady friends while the more polite mainly Japanese crowd around them tried in vain to ignore the scene (on the contrary, I couldn't keep their eyes off them - like I said, I'm pretty certain the ladies were not their girlfriends or wives). As soon as they left, it was so quiet you could almost hear the sigh of relief sweeping over the tables. (On my previous trip, my buddy and I were at the bar every night drinking with an Australian vice cop and singing songs we requested from a pair of guitarists, so we were the loud ones - still, everything closes up by 10 pm or earlier.)
The resort is pretty spread out, so any people noise can easily be avoided by requesting a room away from the main complex, but as far as noisy hotels go, the PPR is pretty tame. Palau is pretty tame for that matter. It's a long trek for Americans and Europeans, so that tends to weed out most casual vacationers.