While you're looking at two very different islands, it's possible for you to get much of what you're looking for at either location.

While it's been a while since I've visited the T&C, if you stay on Grace Bay, you'll be on one of the most beautiful beaches on the Caribbean. Much of what you seek is right along that beach -- restaurants and bars -- and you'll be able to walk along the beach to get to them. I can't speak to the cost of staying there, because it's been too long, but I suspect there is something for every budget.

If you go to St. Martin and stay in Grand Case, you'll find bars and restaurants in walking distance as well, but a different vibe. Grand Case is, effectively, a French village that happens to be on an OK Caribbean beach. One of my favorite Caribbean beach bars, Calmos Cafe, is a walk along the sand. Some of the best restaurants in the Caribbean are also within walking distance, as are a handful of boutiques and shops. The island itself offers plenty more, but it may or may not be your style.

I personally don't like St. Maarten in general, finding it more congested and commercialied than I prefer, but I LOVE Grand Case. I've spent several vacations where I've divided my time between 2-3 days in Grand Case and then the rest of my time on another island (last month, it was Nevis; the time before, it was Anguilla). Which leads me to suggest that you might look into a split as well, splitting between Grand Case and Anguilla -- which has an excellent beach bar culture. Getting to SXM is one of the easier trips to the Caribbean, and Anguilla is a convenient ferry hop over. Anguilla has a justified reputation for being expensive, but depending on the time of year, you can probably find someplace to stay that doesn't stop your heart.

Completely different from each other

If you want one of the worlds most stunning beaches, then TCI hands down

If you need the nightlife, shopping and fun beach bars, then St. Martin

TCI doesn't have much shopping (upscale expensive type) and almost no nightlife, or extremely mellow

St. Martin has the best shopping in Caribbean, tons of fun bars, excellent restaurants, okay beaches. You can day trip to Anguilla for scrumptious beaches one of the days.

But not to confuse you, I have fun at St Martin, but I totally prefer TCI