If the Targa is in great shape I would keep it.
I noticed asking prices for the air-cooled 911's keep on going up and who knows how much they will be if you want to get another one 5-6 years from now.
What kind of environment will the Porsche be kept in?
Here is a thread about winter storage
http://forums.pelicanparts.com/porsche-911-technical-forum/183215-winter-storage-checklist.html
http://forums.pelicanparts.com/porsche-911-technical-forum/386927-very-long-term-storage.html