I cannot decide should I level (i) another Cobalt (eventhough I usually avoid duplicate heros, he is the best Blue attacker and may be worth it), (ii) some other sniper like alice/master lepus, (iii) snow white for her debuff as do not have other blue who does that (iv) isarnia for her deff down (for which curently grim is in place), or (v) none of above and just wait for E&P to give me some miracle elemental down or minion remover.
In my honest opinion, and as others have said, it depends on what you need most for your game.
So master Lepus for your war and raids etc
But Isarnia for your Titan hits
Whichever hero you choose good luck
If you are going to stick with Grimm for Titans, I would go with the Bunny.
I would play with Snow White a little at 70 before deciding to max her. You will probably see that the setup and sacrifice required to get her 420% hit is very underwhelming when compared to other heroes that are faster and\or require no special setup.
I’m gonna stray here and say Snow White. I recently had the same dilemma and went Snow White. With the new raid formations spreading out counter attacks her special is spectacular to see! I haven’t had this much fun since lady Loki. Good luck!