I'd go and get Stoneskin if I was you. Not having it won't be a problem for most fights, but can be quite useful for some others. That, and there's nothing else really useful to cross class.

Mercy Stroke... M'eh. Once again, it's more because there's nothing more to cross class than anything, but I suppose if you want your additionnals 400DMG once a fight, why not.

In the end, you should get both. Mercy Stroke is just much less useful.