i personally think that d90 is good. but it came out last yr & i heard new model coming out. As for canon 550d is a new model & if thinking abt reselling tis 2 550d is much a gd price.
Not that fair to compare D90 with 550D. You may take D5000 to compare with 550D, more appropriate since both cameras are low midrange (doesn't mean you can't take good photos). Not talking about the camera specs which most cameras can take well photos, it's no doubt all depends on practices.
I've been through the same thing you had in mind before, likewise many other CSers had similar experiences for start.
Here's my story. I was thinking to get Nikon D90 first, tired at CP, "Oh nice feeling." Then Canon 550D arrived at IT Show and I play around once. I end up buy Canon 550D after 2nd trial at CP. After few events of shoot, I found the viewfinder/grip too small for me, end up sold 550D away. I waited 2 weeks then finally got my camera. I came from Canon to Nikon because of many other reasons which I will not go into it.
The chase of better and faster dSLR are always never ends. When do you want to start?
Advise is you could test out both cameras at least 2 different times and places (if time permits you). You could pick a shop to go visit first, have a feel and shoot. The personal handling feel is very important especially your hands how big and small it is we can't tell for sure you would like the grip or not.