It's all dependent on skills you use. Projectile damage would be much more valuable for spark users which is spell with projectile tag, but in this case you have critical bonus for attack damage, so the jewel is aimed to be used by all the attack damage types. From bows to quaterstaffs, spears, talismans or maces. Even just for bows they usually would prefer inc attack dmg, since more skills are attacks than just projectiles. It scales heralds for example.
I can't really be more specific because there's so many little variations of builds, but you get the idea. Attack damage has broader range of skills, but it doesn't mean its always the best for everybody. Affix ranges also change and should be higher for more niche type of damage, but I think in this case they both have 1-2%.