The only thing I can say without using both is that the Nikons tend to have less 3rd party software support when it comes to astrophotography although this may be changing these days.
And if it was 150 more then maybe consider looking at the 600D as well?
In truth I think you will be fine with the Nikon if you chose it.
http://snapsort.com/compare/Canon-1100D-vs-Nikon_D3100
http://snapsort.com/compare/Canon-600d-vs-Nikon_D3100