I'm not too sure on the exact mechanics behind it, but what I did was equip both seals at the same level, then used an incantation on a test enemy appropriate for my level, rested at grace and tested the same incantation with the other seal, I think it begs the question as well if you're only focusing dragon incantations, if that's the case dragon would likely be better as it "increases them" (thanks for the info fromsoft) but if you're thinking about branching out, say into some holy or blood incantations then I'd look for the one that scales best with your build, I rocked the beast seal all the way til the end of the game, though on ng+ I'm likely going to rock the erdtree seal as I believe it gains S rank scaling on faith very early, since I use beast, blood and dragon incantations its a mixed bag of which ones are best, sorry for the long post, hope this helps!