Both are guite good, but karin is better in my opinion cause it heals much better and has better control

Karin for sure.

Tsunade is good, slightly overrated. When 6P Naruto gets skill trials, she might be more viable... however Karin heals much more, if you dont kill her with mystery storm, she heals everything up... and her mystery? You either manage to stop her somehow or her team plays with you entire round.

She is most versatile support available, get her... (you can get Tsuna in other events btw... but Karin is Tsuku only ninja so far.)

I disagree with the 2 comments above me. Tsunade is unkillable rounds 1-2 and can revive, plus she heals a lot too, karin just gets more of them. Karin is nice dont get me wrong, but as a support ninja she wont have the most stacked equipment so 1 immunity suppression or someone with higher init who targets karin with their msytery and she is done 2% life recovery on a 3rd or 4th pos ninja isnt really enough to keep them around for long

Tsunade can turn the tide of fights, anecdotal as this is the only karin user I personally have had any trouble with was some s 15* mega whale running madara ten tails/madara gnw/karin,

You have probably seen a lot more tsunades in action than karin, thats for a reason.

Also battlecrier is right that tsunade is in other events, but thats purely because she was released before karin, both will be available in other events sooner or later, but at this stage of the game tsunade is more dependable.

On this topic, does anyone know if karin's bug ever got fixed?

what bug?

karin is for early to mid game and once your power is high its good to switch to tsunade

Because we got some diversity in opinions now, which is great. I will give you some real example which I recently had vs. both Karin and Tsunade while I used same team in both cases. Im using Kisamehada, Taka Suigetsu and Creation Kakuzu with Water main (also in this order based on positions - Kisame P1, Suigetsu P2 ...)
Im in 703k now, which I dont consider mid game anymore.

Vs. Tsunade (team consist of 10T Madara, GNW Madara, Fire Main and Tsunade)
1st round: I use all mysteries, enemy uses 10T Madara.. my team is dead except for Kisame, his team is at 40% except for 10T who is still at full HP.
2nd round: I get hit by 10T again, hit enemy with Kisame and kill fire main... my team ressurects. gets killed right after... at least my Kisame heals up...
3rd round: I get hit by 10T again, hit enemy again and kill his fire main and Tsunade, my team ressurects again... from this moment Tsunade cant do anything... even if she ressurects, she dies instantly. Fight goes on and whether I win or lose is based on my luck with evasion...

Vs. Karin (team consist of 10T Madara, Karin, Fire main and Nagato)
1st round: I interrupt Karin, get hit by Nagato and 10T...
in all other rounds it goes very similar to other fight... my team keeps dying and ressurecting... problem is, I cant kill Karin as easily as Tsunade.
In the end its same luck on evasion based combat where I either dodge and win or dont and lose...

Some comparison of them:
Both Tsunade and Karin heals for roughly 10k per heal.
Tsunade heals twice initially, Karin heals everytime you use mystery.

Tsunade cannot die for 1st 2 rounds.
Karin heals 2% HP per hit... (At my power, she has around 50k - 55k HP at P2 so 1000 - 1100 HP healed per hit)

Tsunade ressurects after you use few mysteries. But usually cannot use mystery if her enemy is counting, so she becomes pretty much useless.
Karin on the other hand deals poison and slow... which basically means, if you cant stop her, you will get interrupted later on. (P1 Karin is annoying)

Now to give you some math:
If you hit Karin with multiple hits (Kisame standard, Minato Standard / mystery etc) she heals multiple times...
If Kisame hits her with standard and deals 10 hits, she heals for 10k, (usually it just lowers total dmg done as Kisame deals more than 1000 dmg per hit)
however if you deal less dmg you are screwed (for example my P3 Kakuzu uses mystery on her). This mystery deals 5 hits, each hit deals 1700 dmg, she heals for 1000 HP per hit and I give her 3500 actual dmg... however she gets extra standard healing for 10k... so my Kakuzu helped Karin to heal.
Basically any mystery that deals less dmg than 11k will help her. Or to give some less specific formula for Karin...
Damage done by mystery must be higher than her heal + 2% of her total HP or you have to burst her before she starts to heal. Otherwise do not use mysteries... you will just help her.

From my experience, Karin is simply stronger as Tsunade helps you only in 1st 2 rounds.

I agree with your case.

They only better use for Tsunade will be if your p2&p3 support her not to be controlled or you are very unbalanced. Then if your p1 clears the field from control you can resurrect your p2&p3 but this starts to sound more situational. So overall Tsunade is more situational then Karin. Also Karin may be used in p1&p2 with good success and in end-game when you will need more balance she will be more viable.

i m not sure if someone can help you at this subject. the opinions are very balanced. all i can say to you is these two things: both are very good units and are worth their prices. what makes them different is the positions that they are placed at. Tsunade is more suitable at pos3 or pos4, while Karin is more suitable for pos2.

have a nice day. i hope that i helped you a bit.