I don't think you need to screen in the lower porch, unless you particularly want to because of mosquitos or whatever.

Which are you more likely to use?

Depends how bad your bugs are! If they're only bad in the evening, pick the one where you will hang out in the evening. If they are bad all the time, do both.

On a lake, screen in both. Bugs come with lake properties so if you want to truly enjoy the space, screen both. If you can't afford to do both at once, then do the one you use the most first and next year the second. You will not regret that at all. Spoken as a true mosquito magnet! lol