I went Core. Although the rule of thumb is to go for Radial unless blaster or scrapper, that doesn't take into account the radius in the proc formula reducing the chance greatly. And someone, Bopper I think, did a study of damage auras and found that they were better off with Core than Radial. I think Oil Slick Arrow, Ignite, and possibly even Full Auto might be better off with Core as well.
Bopper's study also showed that the single target attacks, especially long recharge ones, benefited more from radial than core and that the end result after adding all attacks together was that both sides ended up very close to each other.