From one dyed-in-the-wool grouse hunter to another (truly, a divine madness!), you're right on the money. The other classic American doubles are indeed quite lovely, but the Field-grade Smiths remain the best value.
If it has to be a classic American double for good 'ole Pennsy's Ruffs and Timber-doodles (and you can't afford to break the bank), the Field-grades are about all that remain for the working man. I'd have to give a nod to the 16, but the 20s lighter weight (6-lbs, really?) might make all the difference after a hard-days slog up a grapevine and laurel covered ridge. It sounds like you've done your homework, but make sure it fits well; that will make all the difference. And, for goodness sake, don't drop it on anything too hard.