Originally Posted by woodworkbykirk
the only issue with biscuits is that it may make the jamb too rigid.. with most vinyl windows the extension slot isnt perfect so you sometimes have to manipulate and flex the extension in order to get it into the slot. nails alone will allow it to be adjusted
Not to argue with you, you have great points but the vinyl slot is usually good its the corners that get funky, nice sharp razor knife trims them up nice.
Paint grade I would go with your method
Stain grade, which that looks like, I still would go solid as a rock. Actually I love my joiner so much i would use it for both haha