As far as I can tell, both projectile and whirl finishers yield quite similar effects. The differences are
- the trigger: a projectile is a directed attack that moves through a combo field, while a whirl is an area effect that happens within a combo field;
- the effect target: the projectile finisher affects only the original target of the projectile, while the whirl finisher affects multiple (all?) enemies within an area of effect.
However, the effect of the combo itself is the same for both situations. A poison projectile poisons the affected target, and poison bolts do the same for all of its targets. The same seems to be true for the other combo field types. Hence, if triggered through a light field projectiles remove conditions from adjacent allies, so do the cleansing bolts.
Unfortunately, this is just based on my observations in the BWEs, and I cannot quote a reliable source on this.