What to prioritize at SF seems like a Rorschach test for HD coaches.
In the end I guess SF is the ultimate flex spot ...so it's probably best to find a guy there that complements the strengths and weaknesses of your other players. You can find a wide mix of skillsets in natural SFs, plus you can play SG, PF, and even PG there depending on what you need for the system you run. It's also easy to create mismatches at SF if you have some guys you can move around...
The original question? In a vacuum probably REB>PA, though if I'm going to ask the SF to score a lot at I want him having at least a passable PA rating(so he's not a turnover machine).