I disagree that sheltered plants will give the same produce. At least in the past it was intended that most plants would either give less crops/flowers per harvest when sheltered (I think regardless of season), or it could grow produce only while in season but would then give a better amount. Most of my farming sims grow plants unsheltered, because it's kind of good knowing each specie will take a break. I want to do other stuff with my sims than just gardening. I have no idea which method will produce the most over a full year, but money wise that will of course also depend on quality. Some plants can occasionally get "set back" during freezing temperatures, I had a few Winter plants outdoors producing less on very cold days (can't remember which plant it was, probably a vegetable).
All my experiences were from before the ECO pack, though, things might have changed.
