glad you asked, i own a US ss4 and a SN ss4. i enjoy the SN sub alot more because of its
advantages.
torp speed,torp range, ship speed.
all of these little things are crucial when its time for you to try to win the game for your team.
bascially running from a SN sub isnt easy.if you switch your torps to slow while chasing someone,
it literally feels like they are never going to explode.
The SN sub is also the fastest sub in the game which most people dont know.the speed cap is also
very easy to reach.i go 31/40knots in mine(26/34 underwater).
it turns just as well as the km ss4(i think)

also if you are planning to go far in the SS line, the ss5 is the 2nd best in my opinion.it's one of the few
subs that actually gains tubes instead of losing some from the previous tier.

At lower levels, the IJN and KM subs are preferred. KM is the best sub for sub vs sub combat. The IJN subs are the best BB killers. US subs tend to be fast so they can penetrate farther into enemy territory, UK tend to be hardy. I wouldn't pick a sub line by the SS5 -- it takes way too long to get there. I think you got to decide what you want out it and then pick the line.

I have only played IJN and love it. I'm the type who loves to ruin other people's days with surprises. I get a real joy, stalking the enemy BB5 or 6 and then blasting him with a humongous shot. At times, I've thought about playing KM. They are the most popular and generally most effective because of their speed and proxy torps, but I like the huge punch I carry in IJN.