If you could only have ONE of these DSLR Camera which would you choose?

A) Nikon D90
B) Canon 550d
The Canon has got "Bigger numbers" but does this mean it is a better camera?
Both of these camera have been around a while now the Canon dates back to 2010 and the Nikon goes way back to 2008.
The Canon has the Typical Canon "Kinder egg toy" feel to it. The Nikon D90 feels like it could survive a nuclear blast. That said Nikon has now watered down the build quality on its latest budget DSLR cameras. Like Canon have ALWAYS done.

The D90.

* It is the first camera to offer a video feature.
* It has environmental seals to protect from moisture and dust
* It uses all my Nikkor lenses.
* The D90 is NOT an entry level camera. The camera that replaced it was the D7000 and now the D7100. They too are very robust and have environmental seals

The 550D is a fine camera but not in the same group as the D90.To buy an equivalent Canon, you need to look at the Canon 50D or new 70D… The 60D does not have environmental seals.

I used a D90 before I upgraded to a D7100 (I gave my D90 to my son).It is definitely not a watered down build camera. It is not an entry level camera (as the Canon 550d is), It is an advanced amateur camera. The 550d is more comparable to the Nikon D3100.

I have a friend with a T2i (which is what the 550d is called in the US), and I was not very impressed with it - plastic and cheap feeling.

Even though it is a 2008 design, the D90 still produces great images, is still a very capable camera, and should still perform well.

Comparing the D90 to my D7100, the D7100 has maybe a 20% edge, if that. The advanced cameras are getting so good that each new generation is not really offering much in improvement in base image quality, but rather things like useable ISO and low light capability are extended, which are also important. Dynamic range and other attributes is always improving, but it is in small steps, not giant leaps.

I own a D90, I have used a 550d… Go with the D90.

I was going to say a Hasselblad HD4, but since you limited it to those two, I'd have to go with the Nikon D90.