Well, as these are the two places we've been to multiple times, I feel I can start the responses.
From our perspective they are quite different experiences and vibes. But both are great vacations.
St. Kitts is much more laid back with much less "to do". Things close down early. Not sure that there is any "clubbing". If you want to stay out late, I'm not sure what options you really have. Some our latest nights were at the hotel lobby bar. Kittitian Paradises come to mind...
Aruba has a lot going on. The Palm Beach area is quite built up. You can easily walk to a number of places that are very active - including shopping malls and many chain restaurants. Not so at St. Kitts.
I should say the my wife's favorite by a good margin is St. Kitts. She finds it very much laid back, and she's right. I would prefer Aruba, but only by the slimmest of margins. I'm happy either place.
Toes in the sand, drink in my hand.... ahhhh.
Depending on where you live, Aruba can be very easy to get to. Southwest flies there direct from a number of places (including BWI for us). St. Kitts flights cost a bit more (50% more?) and always require a stop over. Maybe if you're in Miami the differences are less.
Adventure can be different at each. I had a FANTASTIC time last Aruba trip renting ATVs with our youngest son for the day. Drove all around the island (and got all the dust on me to prove it!). Lifetime memory - or at least until I get to do that again. The south side of the island has mountains, pools, natural bridges, etc. ATVing around the Nat'l Park was a real gas. There is hiking around that part of Aruba as well.
St. Kitts has the Brimstone Fortress - a nice half-day trip with history, views and monkeys. There are hiking trails for nice day trips. A nice day at St. Kitts includes walking over to the Carribean side (yes, walking) and enjoying that beach, food and more local island vibe. There are other beaches with fun places to eat and drink for a half or full day.
We've found some nice restaurants at both locations - but it sure feels like there are many more in Aruba. (My favorite one on Aruba is Madame Jeanettes.) There is a classic beach bar right next to the Surf Club - Moomba.
We've rented cars when going to Aruba. Didn't use them every day. We haven't rented at St. Kitts, but rather used local taxis (and once you get a positive relationship with one they'll be available for you just about any time).
The people on both islands are quite nice.
The Aruba Surf Club has the lazy river which can be great if that's of interest to you. Depending on the time of year, you may find many more children at the Surf Club.
Both have their share of chair hogs at the pool, so that's what it is. Never had a problem getting a beach lounge at St. Kitts.
I've rambled enough. If you have specific questions I'll be glad to share my opinion. I'm sure others have much more to share.
Either way - great choices. We enjoy both more than our St. Thomas trip.