Either of those cameras would likely do quite well for you - both are very capable mid-entry cameras with excellent versatility and good features, fast, with good IQ.
You're pretty well fine on brand - they're all fine...just look for the right price, figure out which you are comfortable with in your hands, and see what lenses you can find for each...that should get you started.
Speaking of the A550, I've had much success and have been very pleased with it as a bird/wildlife camera as well as for just about any other type of photography I've done in the past year...paired with a nice long lens it can be a fantastic wildlife cam...much as I'd expect of the D90. Depending on budget, Wolf's right that 300 is a wee bit short for heavy wildlife use, but can get you by pretty well - most likely you'll eventually be wanting 400 or more. Of course, they can get pricey. I've gotten by for years with a very decent Tamron 200-500mm lens, which provides excellent ultrazoom like range (up to 750mm with crop factor), still very strong detail and sharpness with great color, and is cheap comparatively as it doesn't have stabilization or a metal body (Sony DSLR bodies have stabilization, so the lens doesn't need it). For around $800 new, and less used, it's a great lens for wildlife when you need that range. I also picked up a high end Minolta 300mm F4 APO lens for when I need quality over distance...other more expensive options to consider are the Sony 70-400mm which is supposed to be a very good lens.
Nikon has a large lens collection, so you'd likely have no problem finding what you need to match the same on the D90. and Pentax and Canon are also worth a look. Just make sure you like holding the camera in your hand...that's more important than the brand name.
Sony DSLR-A580 / Sony 18-250mm / Minolta 50mm F1.7 / Sigma 30mm F1.4 / Tamron 10-24mm / Tamron 150-600mm / Tamron 90mm F2.8 macro / Minolta 300mm F4 APO
Sony A6000 / 18-55mm F3.5-5.6 / 55-210mm F4-6.3 / 10-18mm F4 / 35mm F1.8 / 16mm F2.8 / via manual adapter, lots of Pentax K mount, Konica K/AR mount, and Leica M mount manual lenses