Knees are stressed a bit more on a bent
though on the flat routes you suggest this will not be a problem.
To get an idea of the difference ride your DF bike with just the
fingertips on the handle bars and see how much harder you have
to push down with the legs compared to when you grip the bars.
This is principally on hills. Watch the play of upper arm muscles on another
rider going up a hill, versus when they are riding flat. Bents don't
permit arms to contribute to propulsion.
My Suede is a "pedal-forward" design so actually no-hands, finger-tips only, or hands fully on makes no changes to me. My knees are not yet good enough to stand on the pedals very well (which is where most arm toquring happens???)
Question - why would there be *more* knee stress if I go with a 'bent (or trike) where the crank is either below or level with the seat?