Was about to ask the same question, Marvaddin.
I did some accurate testings, and I realized that as of now, PvP1 is the best for me, next comes Bronze for the rares. I don't really need money right now, so grinding AI3 isn't that needed, but PvP provides a more risky way to get Electrums, but faster than AI3. Bronze gives me some rares for my decks, since I'm still building up my library.
However, there will be a change, hopefully, I'd want to get into AI4 farming with an upgraded deck. I now have about 10.000 electrums if I sell the upgraded cards I won't need for some time (till when I'll be trying to get all of the cards in the game). I think I'll be farming a little more to have enough money to get an half-unupped AI4 grinder, which one would you recommend me? Consider, too, that since I'm building up my library, I'd rather buy Supernovas/Cremations that have more applications in more decks instead of elite deja vu or ghost of the past that have few to no applications.
(before someone says "build a FG farmer", I did major tests on them, and both unupped or 6-ups weren't good enough for effectively farming, making me lose score, lots of electrums and giving me few possibilities in gaining uppped cards, AI4 is way better for that. I'll eventually build a FG farmer when I'll have lots of upgraded cards and get a fully-upped one, till then, I won't)