i'm push more to the canon because i'v got a tele lens (400mm) but sure if it's worth getting a D3100 any advice please
If you have a 400mm lens for a canon stick with the canon.
As there not much of a second hand market at the moment, I'd have to agree. If the 400mm lens is still in good shape I'd stick with Canon, especially as the 550D is a cracking camera that can be picked up for quite a cheap price at the moment.
550d is really good camera and can got a good price
I like Nikon myself but of the two I would say the 550 for certain. A friend of mine has one and the quality is excellent.