Hey Belle! Definitely a tough decision. I went on several Rustic programs as a high school student, traveled a ton on my own (in Sri Lanka and South East Asia as well) and currently work for the company. Here are my thoughts 
The trip will definitely be different from traveling on your own – you will be with the group the entire time and days are structured with activities and though you will have down time, you won’t be able to leave the group to explore the town on your own. That could definitely be frustrating for someone who has done as much independent travel as you have. I will admit that I recently did an organized travel program (I’m 25) and felt that I was wayyyy too old for it. It was a very “tour bus-y” experience where you get off a bus and get back on (with 45 other people) and was super frustrated with the entire thing because that’s not what I believe traveling should be like! Props to you for thinking about this and doing research before you get on the plane!
Surf and service is one of our base house programs, and generally suggested for first time travelers, so it’s not quite as immersive as some of our other programs. You will work in the community and meet locals, but you will be staying in a house with other teenagers, not in a home stay in a local, very rural village.
However, I really, really believe in the types of travel experiences Rustic creates for its students and travelers. It really is a different experience than traveling to a place on your own, regardless of how much you seek out interacting with the people who live there, because Rustic just has really strong ties in local communities and in places that are hard to access as a traveler on your own.
For example, I went to Fiji last summer for two weeks. Fiji is one of the places we have been for over 20 years – at first I felt that I wanted to go and do my own thing (because I had been to Fiji as a Rustic student and I work for the company!) so for the first week I went off and did my own thing, but the second week I spent with Rustic Pathways up in the village where we work (Nasivikoso) and it was incredible. It was a part of the country I couldn’t access on my own, even though I consider myself a traveler and not a tourist, and always seek out the people and real culture of a place. And I will never forget the people I met. I know that sounds cliche, but it’s true, and I’m sure you can relate to that as someone who has spent so much time in other places and with other people. That’s what makes a Rustic trip especially special.
I would ask you why you specifically chose the surf and service program. If it’s really because you want to learn to surf and work on some service projects then I think you can still have a strong experience even with your extensive independent travel. This won’t be a week-long journey of figuring it out and proving to yourself that you can navigate a country where you don’t speak the language. Everything won’t feel new, or different, or incredibly exciting (some things will, but not everything) but it will be a wonderful way to learn to surf, to meet new people (and be a role model for some of the other students who may be traveling for the fist time), and get an insight into Costa Rica, and know where to go and what to do if you want to come back.
The one thing I can guarantee is that you will meet incredible people – local staff, program leaders, other students. Rustic just attracts very like-minded people who I guarantee you will really bond with.
I would give Rustic a call. You can speak with Wainer (Costa Rica Country Manager) or Ricardo (Costa Rica Country Director) and they can both give you a better idea of whether they think the program choice is right for you. They can also talk through the options you have if you wanted to switch programs for something that is specifically designed for older more experienced students.
Hope that helps!
Best,
Brielle