Hi Lulu, Uvita and Tamarindo are both great places to visit, but they are very different. Tamarindo is more built up, with lots of restaurants and shops in the center of town, and Uvita is more spread out and less developed. The climate differs too- Tamarindo is more arid (tropical dry forest) and Uvita is covered in rainforest and more mountainous. If you have a car, either would be fine but know that in Uvita, you’ll have to drive a little more to get to restaurants, etc. (the Uvita area does have some great places to eat though- especially in Ojochal).
As for what you want to do, there’s great surfing for all levels in both areas, rafting is available in both areas but much closer in Uvita at Rio Savegre and Rio Naranjo, and wildlife viewing/nature is better in Uvita because of all the rainforest. There are lots of nature reserves near Uvita with good walking trails where you can see wildlife like Hacienda Baru. We haven’t made it up there yet but a wildlife sanctuary just opened up in the area at Villas Alturas too- they rescue animals that have been injured in the wild like monkeys. Hope that helps. It really just depends on what kind of experience you want, rainforest and a quieter environment in Uvita or more to do in Tamarindo. If you haven’t seen them yet, we do have a couple of articles on these destinations with more info: The Costa Ballena: Uvita, Dominical, and Ojochal, and Tamarindo: Where Paradise Meets Convenience.
Also, you mentioned getting a rental car. Let us know if you need any help. We get a discount through Adobe Rent a Car of between 10 and 25% (it depends on the type of car and when you’re visiting). There’s more info on our site here, but feel free to ask us a question.