Nikon off course !
You choose Canon 55-250 and Nikon 70-300.
Do you have range limitation? Will a 200mm longer end work for you.
If that is so, why dont you drop 70-300 and get Nikon 55-200 and check if you can get a D7000 instead of D5100.
(70-300 is a very good lens)
Since you are commiting for 3 years, it might make sense to buy a reasonably high end model as of now.
BTW, no matter what, you will buy a lens pretty soon, regardless of what setup you choose.