I've been a photographer for 35 years (some pro but mostly for my own enjoyment). I've owned a lot of cameras in that time, and I've found something good about every camera I've ever owned. The size, weight, ease of loading, type of viewfinder and the list goes on...
For your situation, you've chosen a starting camera that is strong in some areas and not as strong in others. I always find that it's good to learn the strong points of a camera first so I can take advantage of them. If a camera has weaker points, I'll learn how I can work around them and mark that up to something I need to address in my next camera. Simply put, your current camera is known for it's Soft Images. So remember that there are many instances where soft images are desirable and warranted. But you aren't going to get the same shot at the Grand-Prix on a low lit day with a car traveling at 150mph, that you'll get with the Canon XSi or the Nikon 3000.
Before the digital age (now I'm showing my age) we had to deal with film. With film cameras great pictures required great optics. Canon and Nikon have both then and now consistently developed great cameras for every level from beginner to professional. Canon and Nikon also produce lenses of particularly high grade optics to compliment the rugged, intuitive bodies as the years have gone by. Of course, their prices range from $200 to $10,000.
I've always told people who've asked what kind of camera they should buy to ask a few simple questions. What kind of pictures do you take? How often do you use your camera? How much can you afford to spend for a camera? In your case Gregor, I think you've asked these questions since you're looking at high quality entry level cameras developed for the hobbyist or aspiring professional. These cameras
(either one of them) will give you a noticeable boost in sharpness, contrast, and color saturation over your FZ-28.
If I were in your shoes here's what I'd do. First, I'd decide just how dedicated and committed I was to my photography endeavors. Remember that you can spend thousands of dollars on photography even as a hobby and still not have the gear you need to go pro.
Next, I'd use my Panasonic to take as many different types of pictures I could so you can learn simple photographic tenants like depth-of-field, use of hyper-focal distance, how changes in aperture and shutter speed work together with and without flash. The really great thing about digital photography is that you can take as many pictures as you want. Then you can correct your mistakes after you take the pictures (some of them). So take advantage of the full array of features you have with your Panasonic FZ-28 Megazoom.
Once you've done that you'll know what you'd like to have more control over. You'll know how different lenses other than the fixed lens you have now will help you take better pictures. Then, when it comes time to buy that new DSLR, you'll be armed and ready.
I don't think there's anyone that would argue that Canon or Nikon are both great choices. I'm a Canon guy, but that's just because I started with Canon. Remember that once you make your choice it's hard to change because you have an investment in lenses and accessories for the camera you start out with. I would find it hard to sell off all of the great Canon Lenses and bodies I've collected over the years, but it didn't stop me from exploring with other cameras when I had the opportunities.
Gregor, if you truly have a passion for photography all you need to do for now is keep taking pictures. Eventually the pictures will tell you what to do next.