Hello LordVicious! Welcome to the Einaudi thread (assuming this is your first post here, forgive me if I'm mistaken).
Ok, first, I haven't played Berlin Song or Primavera but I have the Islands score book. And I've played I Giorni, Nuvole Bianche (as well as a few others), so I've played a fair amount of Einaudi pieces and I think recently I'm getting a lot better at judging the level of a piece from the score and choosing what to work on based on that. So hopefully my comments will be helpful to you.
Looking at the score for Berlin Song and comparing it to Primavera, if I were choosing for myself I would start with the assumption that Primavera will be easier -- even though there's more going on with the melody etc. and the tempo is much faster. I often find it hard to control my dynamics and phrasing the way I'd like when it's a bunch of repeated block chords like in Berlin Song. Whereas in Primavera, it's all single notes (ok, not all, but any time there's movement, it's all single notes).
Primavera might be harder to work up initially and to get going, but I would guess (again, just from looking at the score) that it would be the easier one to polish after getting it into the fingers.
Hmmm, now I'm thinking I want to start working on Primavera...
IIRC Monica has played Primavera, so maybe she'll share her thoughts.
Also, you mentioned you have to choose by Thursday, is that a lesson? If so, why know ask your teacher's advice? (If not, please disregard!)
Keep us posted about what you choose and how you get on!