Hemlock from what I read does not take preservatives well and it is not a
top rated exterior wood.  (There is usually a reason why a material is not
used for the previous two hundred years!)  I have seen Poplar rot very
quickly, on the order of #2 pine.  Regardless of preparations and coatings a
vertical grain mahogany, redwood, cypress or cedar (most species) will last
much longer than pine or poplar.  Now, which is better, old growth pacific
hemlock or brownstone?
It is good to hear all this wood talk.
Best,
Leland