Start by identifying what your 550D is not doing well enough. Upgrade the body only when you can name a real limitation, such as autofocus speed/accuracy, frame rate for sports or wildlife, or cleaner high-ISO performance in low light.

For many photographers, lenses make a bigger difference than a body upgrade. A strong macro lens, a long lens for wildlife, or faster glass can improve sharpness, contrast, and usability more than a new camera body. Lighting gear can also have a huge impact, especially for portraits and close-up work.

If you do move up, higher-end bodies mainly help you work faster and more confidently. For your subjects, autofocus performance and burst rate are often more important than simply getting more resolution.

Brand choice matters most once you start building a lens kit. If you are not invested yet, compare systems based on the lenses and features you actually need. If you already plan to buy Canon lenses, staying with Canon usually makes the path simpler.

In short: define your current camera’s limits first, prioritize lenses and lighting unless the body is clearly holding you back, and choose a brand based on the lens ecosystem and features that support your subjects.