Both are great choices. I have found more to do in terms of activity in Aruba. Water color is beautiful in T&C. I would be careful of weather that time of year. Aruba would be the safest bet as it is outside the hurricane zone. I would also consider where you are flying from, and the time it takes as you are going for 5 days. Dependent on those 2 factors you can make the choice. Both are top notch places and you can jet ski in either. I am partial to Aruba personally but others on these boards love T&C. I have been to both my vote ultimately is for Aruba mostly due to weather concerns in August/September. Get insurance if you choose T&C.

First off, Aruba does get hurricanes! Same chances of weather everywhere, so if we could predict the weather we would be worth a fortune!

TCI is a dry dessert island, Aruba is too, but it can get wash out rain days and is windy.

Aruba is fun, lots to do, active, nightlife.
TCI is laid back quiet and great for relaxing. (although water sports including jet ski are plenty)

TCI is much closer to get to, especially for only 4 nights, Aruba you'll loose a day coming and going

The water in TCI is the prettiest in the world.

Since what you want is to relax, enjoy the beach and water sports for only 4 nights, I would pick Turks and Caicos.

Both destinations are quite different. Aruba is very built up and commercialized. Most people do not like that aspect of it. On the contrary, that is exactly why we love it. So many things to see and do and tons of restaurants and bars. We like to be very active on vacation, so Aruba works well with keeping busy.

The Palm beach area has highrise hotels lined up beach front in a row. On the beach side are water sport stations, bars and restaurants. On the street side within walking distance are tons of restaurants, bars, shopping and entertainment. It kinda reminds me of Las Vegas with a beach. Everything is just right there.

The Eagle Beach area has low rise hotels. The beach is wider, more spread out and usually less crowded. Not as many restuarants or bars in that area, but the local bus is convenient to get to Palm Beach or downtown. I do not remember seeing any jet skis in Aruba.

Here is a link to the main tour operator in Aruba. It will give you a general idea of things to see and do:

http://www.depalmtours.com/

The draw to Turks & Caicos is of course Grace Bay Beach. It is absolutely gorgeous. If your idea of a beach vacation is quite and relaxed on a sugar white sand beach with crystal clear water, than T & C is the destination. There is not much there to do besides water sports. Snorkeling, sailing, scuba diving, semi-sub and yes, there is a fabulous jet ski tour. Everything here is really spread out, so a car is necessary to get to other areas and dining options.

Here's the link for the jet ski tour in T & C. Make sure if you do this that you do the 2 hour guided tour and not just the rent by the hour.

http://www.tcimall.tc/sunandfun/

I agree with diann24 regarding time of year. For that time of year Aruba is the safer bet weather wise. However, for Aruba you really need 7 - 10 days to get the most out of your vacation. Hurricane/trip interuption insurance is a must. Just make sure it can be canceled for any reason and not just if your flight is canceled or hotel closes.

I am not sure why you tagged the USVI. It is not a big jet ski destination. They are available, but most are guided tours because of liability issues. If you would like other water activities such as sailing, snorkeling, SUP, island hopping, you might consider the USVI.