Your iPhone 6S has a much better camera, YES. Unless you just absolutely want to carry another camera, you definitely don't need anything but the iPhone. Even if you're a professional photographer who has to have a DSLR everywhere, you'll still be pretty pleased with the shots you get on the phone (not to mention the convenience of not carrying around a big ol' camera).
Also, the battery life on the iPhone 6S is excellent. Even if you take a ton of pics, you likely won't need to charge it until the evening.