Thanks for replying!
The nodes in the blessing tree which use breaths to upgrade are very clear and easy to see what changed (although I wish I could see how much the numbers will increase before I buy the first upgrade).
The ones I'm wondering about are the active abilities in the skills tab, pictured here. For example I have level 7 Blades of Contrition, but how is it different from level 6 Blades of Contrition? I guess it does more damage but I have no idea how much more damage, which makes it hard to decide stuff like "ok should I use my next 5sp on upgrading Blades of Contrition or should I upgrade Holy Flame instead?".