GHHalley said it all.

In general you can go faster,further, with bigger possibilities to catch enemy ships with
enough air to survive, before you hit the bottom of the ocean.

I stuck with Junsen until weight became an issue. The Kadai is faster and more
manuverable, and the torps hit harder and have greater range, but I really missed
the 2 extra tubes in the front.

For ASW the Kadai is much better, especially when using the tactics in the SS guide
where you hit the enemy with your rear tubes. Then the speed, the manuverability,
and the torpedos make a significant difference.

"well my engies are at lvl 65 only.so i can stick in junsen until i run out of displacement? "

The point is, that having 4 support slots rather than 3 means 2 engineers instead of 1
(assuming you actually use your sonar and planesman). 2 engineers will give you more OH
time, and as long as they are roughly equal in true ability the speed will stay the same.

I'd recommend at least taking the kaidai out for a few games to form your own
opinion, but you can do whatever you want.

well my engies are at lvl 65 only.so i can stick in junsen until i run out of displacement?

Extra support slot (same number of sailors but rearranged). Ability to run 2 engineers
with planesman and sonar. Longer OH time and depending on Engineers abilities easier to cap.

Mainly it will allow you to stay underwater longer and a bit more durable. I'm not
sure, but it prolly goes a bit faster than the junsen as well. You do lose a bit of front
end punch, but overall a better sub.