assuming they are a similar price they will both be able to do the job, the question is which ecosystem are you buying into? once you start down the route of canon you are stuck with canon and once you start with nikon you are stuck with nikon as the lenses are not interchangeable.
Have a look at the reviews on dpreview, they are very indepth and it might be that for this generation one is actually 'better' than the other.