My initial thought is that you have all three of the pulverizers, Zimkitha will hit the enemy hard when under their debuff. Helps that she is fast and if you have another red on the team the elemental link heal is very, very helpful.
As Kerridoc said, she overwrites the attack buff from Boldtusk so watch the firing order if running them both.
I don’t have Mitsuko so don’t have experience with her. Zimkitha is a mainstay on my team and I find her to be fantastic (defense and raiding).
Max Mitsy, since Zimi cleanses also well @70. Zimi doesn’t hit hard, no matter if maxed or 3rd tier. I’d max Mitsi, since she’s the rarer one.
I’ve got Mitsuko and I agree with Kerridoc on that point. Although her real strength and niche is in dealing with strong offensive Ice heroes with powerful skills, Mitsuko’s mana control makes her more versatile and a good addition to your bench. In wars she is invaluable, I’ve got her at 2/60 as well and even at this level she’s pretty useful, her Mirror of Flames helps demolish a fully maxed Grimm or Isarnia, and after those are done, she can still help stop a dangerous foe about to fire their skill.
I was in a similar situation a while back. I went with Mitsuko, mostly because I was lacking good sorcerers. I don’t regret it one bit, she’s an amazing hero. I like to pair her with a mana freezer like Hel or Proteus. Since their specials won’t stop an enemy from firing if their mana is already charged, Mitsuko’s mana reduction is invaluable. Zim is a great utility hero, but I view her as sort of a jack of all trades, master of none.
Plus, it’s fun to watch a blue enemy obliterate themselves when Mitsuko’s special is active. 
I went with Mits over Zim and really I do regret it.
Zims heal link is nice and stacks with other ‘heal over time’ reds you may use such as Red Hood and Ares. People turn their nose up at her weak AoE damage and attack boost being low when compared with other heroes, but Mits is hardly a wrecking ball either in terms of spec damage and a +25% boost is better than no boost. Yes there are other cleansing heroes, but sometimes Rigard isn’t the ideal choice, and a fast charging cleanse is exceptional.
Mits gets some redemption in being the only fire hero with any mana control but it’s not that important, and when her main use is to bait blue enemies into kamikaze dives, you don’t really want to prolong their attack anyway: the reflect is active as soon as she hits, so let them fire at her immediately. In my subjective view, her mana control is the least effective after Chao. In diamond arena, the reflection is fun but heroes like Richard, Thorne, Athena, Isarnia just aren’t enough of a threat to worry about anyway, and Alasie, Magni, Alice have often charged and struck before Mits is ready.
For titan fights, the mana control is OK but the reflect is useless, I would rather have Zims higher attack stat, fast cleanse, attack boost and heal any day.
TLDR; Zim is a boring yet useful multi-tool hero, Mits is niche and unnecessary.
I don’t have Mits. But I do love my Zim. Complete utility hero, no real strength but useful everywhere. When I see a defense with Mits, I don’t bring a blue and she no longer matters. If you need a sorcerer more than a druid, that is something to consider as well.
Overwriting ability might be Zim’s only downside, so triggering abilities in the best order requires thought.
I ascended Zim over Mitsuko (only other fully ascended red is Jolly St Nick) and I’m glad I did.
Zim is useful in almost all situations. That’s why she got the rings. Mitsuko is an awesome hero, but I just wouldn’t get as much use. A Zim and Santa stack, for me, was better than a Mit and Santa.
Also, Mitsuko does just fine in the Trials at 3/70.
I went with Zim too even though I have a maxed Kage and Inari which would give me a 4% Sakura mana bonus.
In diamond, just too many GMs and having a fast cleanse that can survive a couple of hits is so useful.
My heart screams MITS! but my mind says Zim… Especially that I’m strictly f2p and Zim gives me bigger bang per buck.
Thought about it a lot and this is what I came up with (thanks for help everyone!). Maybe will be useful to anyone else being in a pickle:
Pro Zim:
Con Zim:
Pro Mits:
Con Mits:
If I forgot something important or you don’t agree, let me know.
Cheers!
I’ll second her utility on 8-7 autofarming haha, it’s the best (even at 3-70 where I have her). She’s very durable when maxed but I still wonder whether I want to spend the rings on her because she’s so darn boring to play with and her special doesn’t really hit that hard against tougher opponents.
Autofarming should never ever be a reason to recommend a hero. A team of fully ascended 3 stars can autofarm most relevant stages.
I do want to add though that there’s a reason I personally still have Mits at 2/60, it’s because her strength is niche and because it can work at any level (that and I just thought Red Hood was cuter lol). Zim can too, but her strength is a lot more versatile. I think eventually you will want to ascend both, but I’d do Zim first myself. If you eventually place both in defense (Mitsuko center and Zim in the corner) both will play off each other, and your opponents will have to think twice about bringing offensive blue heroes.
Zim any day over Mitsuko.
When you level more heroes in your roster, you will need more space for other heroes in your lineup. Zim can replace both BT and Rigard (also fast mana) and you can save one spot for another hero.
Mitsuko in my opinion is very niche. Nice to have but not necessary. She is my 4th red.
I went with Zim first. Her versatility is amazing. She self heals (even if its just a small percentage), cleanses and adds an attack buff. No, she doesn’t hit hard, but you will use her in more ares then you will Mits. Mits works great at 3/70. You need the extra leveling for Zim to get her full potential.
I vote for Zim.
Really good hero for all the jobs.
Mitsuko is more niche.
My Zimkitty is 4/80 and has 4 talents, and my Mits is 3/70 and rarely sees the battle at all. The only place I use Mits is in events/Atlantis levels when the boss is blue.
Mits’ special kinda works against itself, mana cut prevents the boss from firing, and you’ll need to throw more tiles at the boss to finally trigger the special. But of course, blue special mirroring is VERY satisfying when it finally happens. So, Mits is a very cool, but has the niche that is too narrow.
Don’t have mitsuko, got Zim with 2 talents.
I find her awesome. If her card at start was a bit underwelming compared to other 5* (no big damage, no big buff, okayish damage at best), after having to play with her it’s paradise on earth.
Having a 5* cleanser is so huge that every GM or isarnia tanks are just looking like free raids (2250-2400 cups)
I’d be only happy to have mistuko for wars, where she seems to be a monster.
Love playing with Mitsuko. Really love ZimKitty’s design but her attacks leave me yawning. I use her for wars and that’s it.
Oh, I don’t like leaving Zim behind at all. She is at +6 going attack/defense both times. And when she gets a little thorny, watch out. That silly druid ability makes her quite durable.
Zimkitha is tough, she can take a direct, maxed sniper at least once (two if you’ve blunted their attack), the team cleanse makes her indispensable. She may be a utility hero but what great utility.