All the canoes in the Bell Northstar-Northwind-Northwoods line have tremendous secondary stability (stability when the canoe is in motion) so, comparing apples to apples, that is the Northstar (16-6) to the Northwoods (18-6), I don't think you'd find much difference in stability.  The bigger canoe will undoubtedly be a hair slower due to the load you'll be carrying (although the hull speed is actually higher) but, given its added length, should track a little better.
Congratulations on your new canoe.  If you haven't paddled a Bell before, you're going to love it.  Hang on to it, those Bells are getting rarer as we speak!